diff --git a/CHANGELOG.md b/CHANGELOG.md
index b4eea0e57..158f6e24c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@
 
 * **Update Python SDK usage in `partition_via_api`.** Make a minor syntax change to ensure forward compatibility with the upcoming 0.26.0 Python SDK.
 * **Remove "unused" `date_from_file_object` parameter.** As part of simplifying partitioning parameter set, remove `date_from_file_object` parameter. A file object does not have a last-modified date attribute so can never give a useful value. When a file-object is used as the document source (such as in Unstructured API) the last-modified date must come from the `metadata_last_modified` argument.
+* **Fix occasional `KeyError` when mapping parent ids to hash ids.** Occasionally the input elements into `assign_and_map_hash_ids` can contain duplicated element instances, which lead to error when mapping parent id.
 
 ## 0.15.13
 
diff --git a/test_unstructured/documents/test_elements.py b/test_unstructured/documents/test_elements.py
index 7b3ff7360..298d6bed8 100644
--- a/test_unstructured/documents/test_elements.py
+++ b/test_unstructured/documents/test_elements.py
@@ -704,6 +704,21 @@ def test_hash_ids_are_unique_for_duplicate_elements():
             ), "Parent ID hasn't changed after recalculation"
 
 
+def test_hash_ids_can_handle_duplicated_element_instances():
+    # GIVEN
+    parent = Text(text="Parent", metadata=ElementMetadata(page_number=1))
+    element = Text(text="Element", metadata=ElementMetadata(page_number=1, parent_id=parent.id))
+    elements = [parent, element, element]
+
+    # WHEN
+    updated_elements = assign_and_map_hash_ids(copy.deepcopy(elements))
+    ids = [element.id for element in updated_elements]
+
+    # THEN
+    assert len(ids) == len(set(ids)) + 1, "One element is duplicated so uniques should be one less."
+    assert elements[1].metadata.parent_id == elements[2].metadata.parent_id
+
+
 def test_hash_ids_are_deterministic():
     parent = Text(text="Parent", metadata=ElementMetadata(page_number=1))
     elements = [
diff --git a/unstructured/documents/elements.py b/unstructured/documents/elements.py
index c1b29ee6d..07971b806 100644
--- a/unstructured/documents/elements.py
+++ b/unstructured/documents/elements.py
@@ -562,7 +562,7 @@ def assign_and_map_hash_ids(elements: list[Element]) -> list[Element]:
     # -- map old parent IDs to new ones --
     for e in elements:
         parent_id = e.metadata.parent_id
-        if not parent_id:
+        if not parent_id or parent_id not in old_to_new_mapping:
             continue
         e.metadata.parent_id = old_to_new_mapping[parent_id]