diff --git a/docarray/array/storage/base/getsetdel.py b/docarray/array/storage/base/getsetdel.py
index be10730ce74..3b38658e107 100644
--- a/docarray/array/storage/base/getsetdel.py
+++ b/docarray/array/storage/base/getsetdel.py
@@ -235,6 +235,10 @@ def _set_doc_attr_by_offset(self, offset: int, attr: str, value: Any):
         :param attr: the attribute of document to update
         :param value: the value doc's attr will be updated to
         """
+        if attr == 'id' and value is None:
+            raise ValueError(
+                'setting the ID of a Document stored in a DocumentArray to None is not allowed'
+            )
         _id = self._offset2ids.get_id(offset)
         d = self._get_doc_by_id(_id)
         if hasattr(d, attr):
diff --git a/docarray/array/storage/memory/backend.py b/docarray/array/storage/memory/backend.py
index 804320343f9..773289ec621 100644
--- a/docarray/array/storage/memory/backend.py
+++ b/docarray/array/storage/memory/backend.py
@@ -1,7 +1,10 @@
+import functools
 from typing import (
     Optional,
     TYPE_CHECKING,
     Iterable,
+    Callable,
+    Dict,
 )
 
 from ..base.backend import BaseBackendMixin
@@ -13,9 +16,44 @@
     )
 
 
+def needs_id2offset_rebuild(func) -> Callable:
+    # self._id2offset needs to be rebuilt after every insert or delete
+    # this flag allows to do it lazily and cache the result
+    @functools.wraps(func)
+    def wrapper(self, *args, **kwargs):
+        self._needs_id2offset_rebuild = True
+        return func(self, *args, **kwargs)
+
+    return wrapper
+
+
 class BackendMixin(BaseBackendMixin):
     """Provide necessary functions to enable this storage backend."""
 
+    @property
+    def _id2offset(self) -> Dict[str, int]:
+        """Return the `_id_to_index` map
+
+        :return: a Python dict.
+        """
+        if self._needs_id2offset_rebuild:
+            self._rebuild_id2offset()
+
+        return self._id_to_index
+
+    def _rebuild_id2offset(self) -> None:
+        """Update the id_to_index map by enumerating all Documents in self._data.
+
+        Very costy! Only use this function when self._data is dramtically changed.
+        """
+
+        self._id_to_index = {
+            d.id: i for i, d in enumerate(self._data)
+        }  # type: Dict[str, int]
+
+        self._needs_id2offset_rebuild = False
+
+    @needs_id2offset_rebuild
     def _init_storage(
         self,
         _docs: Optional['DocumentArraySourceType'] = None,
@@ -23,9 +61,12 @@ def _init_storage(
         *args,
         **kwargs
     ):
+        from docarray.array.memory import DocumentArrayInMemory
+
         super()._init_storage(_docs, copy=copy, *args, **kwargs)
 
-        self._data = {}
+        self._data = []
+        self._id_to_index = {}
         if _docs is None:
             return
         elif isinstance(
@@ -33,8 +74,11 @@ def _init_storage(
             Iterable,
         ):
             if copy:
-                for doc in _docs:
-                    self.append(Document(doc, copy=True))
+                self._data = [Document(d, copy=True) for d in _docs]
+            elif isinstance(_docs, DocumentArrayInMemory):
+                self._data = _docs._data
+                self._id_to_index = _docs._id2offset
+                self._needs_id2offset_rebuild = _docs._needs_id2offset_rebuild
             else:
                 self.extend(_docs)
         else:
diff --git a/docarray/array/storage/memory/getsetdel.py b/docarray/array/storage/memory/getsetdel.py
index 55f7865499d..993a5e1dce5 100644
--- a/docarray/array/storage/memory/getsetdel.py
+++ b/docarray/array/storage/memory/getsetdel.py
@@ -1,43 +1,75 @@
+import itertools
 from typing import (
     Sequence,
     Iterable,
+    Any,
 )
 
 from ..base.getsetdel import BaseGetSetDelMixin
-from ..base.helper import Offset2ID
+from ..memory.backend import needs_id2offset_rebuild
 from .... import Document
 
 
 class GetSetDelMixin(BaseGetSetDelMixin):
     """Implement required and derived functions that power `getitem`, `setitem`, `delitem`"""
 
+    @needs_id2offset_rebuild
+    def _del_docs_by_mask(self, mask: Sequence[bool]):
+        self._data = list(itertools.compress(self._data, (not _i for _i in mask)))
+
+    @needs_id2offset_rebuild
+    def _del_docs_by_slice(self, _slice: slice):
+        del self._data[_slice]
+
     def _del_doc_by_id(self, _id: str):
-        del self._data[_id]
+        self._del_doc_by_offset(self._id2offset[_id])
+
+    @needs_id2offset_rebuild
+    def _del_doc_by_offset(self, offset: int):
+        del self._data[offset]
+
+    def _set_doc_by_offset(self, offset: int, value: 'Document'):
+        old_id = self._data[offset].id
+        self._id2offset[value.id] = offset
+        self._data[offset] = value
+        self._id2offset.pop(old_id)
 
     def _set_doc_by_id(self, _id: str, value: 'Document'):
-        if _id != value.id:
-            del self._data[_id]
-        self._data[value.id] = value
+        old_idx = self._id2offset.pop(_id)
+        self._data[old_idx] = value
+        self._id2offset[value.id] = old_idx
 
-    def _set_doc_value_pairs(
-        self, docs: Iterable['Document'], values: Sequence['Document']
-    ):
-        docs = list(docs)
+    @needs_id2offset_rebuild
+    def _set_docs_by_slice(self, _slice: slice, value: Sequence['Document']):
+        self._data[_slice] = value
 
-        for _d, _v in zip(docs, values):
-            _d._data = _v._data
+    def _set_doc_attr_by_offset(self, offset: int, attr: str, value: Any):
+        if attr == 'id' and value is None:
+            raise ValueError(
+                'setting the ID of a Document stored in a DocumentArray to None is not allowed'
+            )
+
+        setattr(self._data[offset], attr, value)
+
+    def _get_doc_by_offset(self, offset: int) -> 'Document':
+        return self._data[offset]
 
     def _get_doc_by_id(self, _id: str) -> 'Document':
-        return self._data[_id]
+        return self._data[self._id2offset[_id]]
 
-    def _get_docs_by_ids(self, ids: Sequence[str]) -> Iterable['Document']:
-        return (self._data[_id] for _id in ids)
+    def _get_docs_by_slice(self, _slice: slice) -> Iterable['Document']:
+        return self._data[_slice]
 
     def _clear_storage(self):
         self._data.clear()
+        self._id2offset.clear()
 
     def _load_offset2ids(self):
-        self._offset2ids = Offset2ID()
+        ...
 
     def _save_offset2ids(self):
         ...
+
+    _set_doc = _set_doc_by_id
+    _del_doc = _del_doc_by_id
+    _del_all_docs = _clear_storage
diff --git a/docarray/array/storage/memory/seqlike.py b/docarray/array/storage/memory/seqlike.py
index e210f521e1a..061382ab910 100644
--- a/docarray/array/storage/memory/seqlike.py
+++ b/docarray/array/storage/memory/seqlike.py
@@ -1,4 +1,6 @@
-from typing import Union, Iterable
+from typing import Union, Iterable, MutableSequence, Iterator
+
+from ..memory.backend import needs_id2offset_rebuild
 
 from ..base.seqlike import BaseSequenceLikeMixin
 from .... import Document
@@ -7,19 +9,42 @@
 class SequenceLikeMixin(BaseSequenceLikeMixin):
     """Implement sequence-like methods"""
 
+    @needs_id2offset_rebuild
+    def insert(self, index: int, value: 'Document'):
+        """Insert `doc` at `index`.
+
+        :param index: Position of the insertion.
+        :param value: The doc needs to be inserted.
+        """
+        self._data.insert(index, value)
+
+    def append(self, value: 'Document'):
+        """Append `doc` to the end of the array.
+
+        :param value: The doc needs to be appended.
+        """
+        self._data.append(value)
+        if not self._needs_id2offset_rebuild:
+            self._id_to_index[value.id] = len(self) - 1
+
     def __eq__(self, other):
         return (
             type(self) is type(other)
             and type(self._data) is type(other._data)
             and self._data == other._data
-            and self._offset2ids == other._offset2ids
         )
 
+    def __len__(self):
+        return len(self._data)
+
+    def __iter__(self) -> Iterator['Document']:
+        yield from self._data
+
     def __contains__(self, x: Union[str, 'Document']):
         if isinstance(x, str):
-            return x in self._data
+            return x in self._id2offset
         elif isinstance(x, Document):
-            return x.id in self._data
+            return x.id in self._id2offset
         else:
             return False
 
@@ -30,3 +55,9 @@ def __add__(self, other: Union['Document', Iterable['Document']]):
         v = type(self)(self)
         v.extend(other)
         return v
+
+    def extend(self, values: Iterable['Document']) -> None:
+        values = list(values)  # consume the iterator only once
+        last_idx = len(self._id2offset)
+        self._data.extend(values)
+        self._id_to_index.update({d.id: i + last_idx for i, d in enumerate(values)})
diff --git a/tests/unit/array/mixins/test_traverse.py b/tests/unit/array/mixins/test_traverse.py
index c983453fa74..0a128551de2 100644
--- a/tests/unit/array/mixins/test_traverse.py
+++ b/tests/unit/array/mixins/test_traverse.py
@@ -736,3 +736,27 @@ def test_traverse_flat_offset():
     assert len(flat_docs) == 2
     assert flat_docs[0].id == 'r2c1'
     assert flat_docs[1].id == 'r2c2'
+
+
+def test_traverse_flat_conflicting_ids():
+    da = DocumentArray(
+        [
+            Document(
+                id=f'r{i}',
+                chunks=[Document(id=f'rc{j}') for j in range(3)],
+                matches=[Document(id=f'rm{j}') for j in range(3)],
+            )
+            for i in range(3)
+        ]
+    )
+
+    for traversal_path in ['rc', 'rm']:
+
+        flattened = da.traverse_flat(traversal_path)
+        assert len(flattened) == 9
+        child_ids = set()
+
+        for child in flattened:
+            child_ids.add(id(child))
+
+        assert len(child_ids) == 9
diff --git a/tests/unit/array/test_advance_indexing.py b/tests/unit/array/test_advance_indexing.py
index bad7725606e..87b37a7ee8d 100644
--- a/tests/unit/array/test_advance_indexing.py
+++ b/tests/unit/array/test_advance_indexing.py
@@ -635,3 +635,26 @@ def test_offset2ids_persistence(storage, config, start_storage):
     da = DocumentArray(storage=storage, config=config)
 
     assert da[:, 'id'] == da_ids
+
+
+def test_dam_conflicting_ids():
+    docs = [
+        Document(id='1'),
+        Document(id='2'),
+        Document(id='3'),
+    ]
+
+    d = Document(id='1')
+    da = DocumentArray()
+    da.extend(docs)
+    da.append(d)
+
+    assert len(da) == 4
+    assert id(da[0]) == id(docs[0])
+    assert id(da[3]) == id(d)
+
+    da[0].text = 'd1'
+    da[3].text = 'd2'
+
+    assert docs[0].text == 'd1'
+    assert d.text == 'd2'
diff --git a/tests/unit/array/test_base_getsetdel.py b/tests/unit/array/test_base_getsetdel.py
index 146296fad76..c0ef9f2a4f6 100644
--- a/tests/unit/array/test_base_getsetdel.py
+++ b/tests/unit/array/test_base_getsetdel.py
@@ -1,10 +1,10 @@
 from abc import ABC
+from typing import Iterable, Sequence
 
 import pytest
 
 from docarray import DocumentArray, Document
 from docarray.array.storage.base.getsetdel import BaseGetSetDelMixin
-from docarray.array.storage.base.helper import Offset2ID
 from docarray.array.storage.memory import BackendMixin, SequenceLikeMixin
 
 
@@ -14,18 +14,33 @@ class DummyGetSetDelMixin(BaseGetSetDelMixin):
     # essentials
 
     def _del_doc_by_id(self, _id: str):
-        del self._data[_id]
+        del self._data[self._id2offset[_id]]
+        self._id2offset.pop(_id)
+
+    def _del_doc_by_offset(self, offset: int):
+        self._id2offset.pop(self._data[offset].id)
+        del self._data[offset]
 
     def _set_doc_by_id(self, _id: str, value: 'Document'):
-        if _id != value.id:
-            del self._data[_id]
-        self._data[value.id] = value
+        old_idx = self._id2offset.pop(_id)
+        self._data[old_idx] = value
+        self._id2offset[value.id] = old_idx
+
+    def _get_doc_by_offset(self, offset: int) -> 'Document':
+        return self._data[offset]
 
     def _get_doc_by_id(self, _id: str) -> 'Document':
-        return self._data[_id]
+        return self._data[self._id2offset[_id]]
+
+    def _set_doc_by_offset(self, offset: int, value: 'Document'):
+        self._data[offset] = value
+        self._id2offset[value.id] = offset
 
-    def _clear_storage(self):
-        self._data.clear()
+    def _get_docs_by_slice(self, _slice: slice) -> Iterable['Document']:
+        return self._data[_slice]
+
+    def _set_docs_by_slice(self, _slice: slice, value: Sequence['Document']):
+        self._data[_slice] = value
 
 
 class StorageMixins(BackendMixin, DummyGetSetDelMixin, SequenceLikeMixin, ABC):
@@ -37,7 +52,7 @@ def __new__(cls, *args, **kwargs):
         return super().__new__(cls)
 
     def _load_offset2ids(self):
-        self._offset2ids = Offset2ID()
+        pass
 
     def _save_offset2ids(self):
         pass
diff --git a/tests/unit/document/test_feature_hashing.py b/tests/unit/document/test_feature_hashing.py
index ec112c5c5fd..5702678ac8f 100644
--- a/tests/unit/document/test_feature_hashing.py
+++ b/tests/unit/document/test_feature_hashing.py
@@ -21,6 +21,7 @@ def test_feature_hashing(n_dim, sparse, metric):
     assert da.embeddings.shape == (6, n_dim)
     da.embeddings = to_numpy_array(da.embeddings)
     da.match(da, metric=metric, use_scipy=True)
-    for doc in da:
-        assert doc.matches[0].scores[metric].value == pytest.approx(0.0)
-        assert doc.matches[1].scores[metric].value > 0.0
+    result = da['@m', ('id', f'scores__{metric}__value')]
+    assert len(result) == 2
+    assert result[1][0] == 0.0
+    assert result[1][1] > 0.0