fix: preserve duplicate take offsets (#4024)

## Summary
- preserve repeated table offsets without adding a public ordering
guarantee
- retain exact requested ordering in identity and persisted permutations
- cover local, projected, multi-batch, and mocked-remote query paths

## Root cause
Take queries lowered offsets to a set-like IN predicate and discarded
repeated occurrences. Persisted permutation loading also compared the
distinct base-table result count with the requested occurrence count,
rejecting repeated row IDs before its existing reordering step could
expand them.

## Fix
The shared take-query path now deduplicates the predicate for efficient
lookup, requests row-offset metadata internally, and expands each
matching row to the requested multiplicity in backend result order. An
internal opt-in keeps exact requested order for identity
PermutationReader reads, while persisted permutations continue using
their existing ordering map.

## Validation
- cargo test --quiet --features remote --tests
- cargo check --quiet --features remote --tests --examples
- cargo clippy --quiet --features remote --tests --examples
- targeted Python local and mocked-remote regression tests
- exact issue reproduction

Fixes #2820

<!-- lance-gatekeeper-fix:v1 agent=75acf840afa6f4be4bff98b567b504bd
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>
Co-authored-by: Xuanwo <github@xuanwo.io>

python/python/tests/test_query.py

@@ -1923,6 +1923,21 @@ def test_take_queries(tmp_path):
         17,
     ]
 
+    # Duplicate offsets are occurrences, not set members. Ordering is unspecified.
+    assert sorted(table.take_offsets([5, 2, 5, 17]).to_pandas()["idx"].to_list()) == [
+        2,
+        5,
+        5,
+        17,
+    ]
+
+    # Converting a take builder to its serializable query representation must
+    # retain occurrence metadata and execute with the same multiplicity.
+    query = table.take_offsets([5, 2, 5, 17]).select(["idx"]).to_query_object()
+    assert query.take_offsets == [5, 2, 5, 17]
+    converted = table._execute_query(query).read_all()
+    assert sorted(converted["idx"].to_pylist()) == [2, 5, 5, 17]
+
     # Take by row id
     assert list(
         sorted(table.take_row_ids([5, 2, 17]).to_pandas()["idx"].to_list())

python/python/tests/test_remote_db.py

@@ -479,24 +479,49 @@ def test_remote_permutation_is_picklable():
                 match = re.search(
                     r"_rowoffset\s+in\s+\((.*?)\)", body["filter"], re.IGNORECASE
                 )
-                offsets = [int(o.strip()) for o in match.group(1).split(",")]
+                offsets = list(
+                    dict.fromkeys(int(o.strip()) for o in match.group(1).split(","))
+                )
             else:
                 offsets = list(range(len(rows)))
-            table = pa.table({"a": [rows[offset] for offset in offsets]})
+            columns = body.get("columns") or ["a"]
+            table = pa.table(
+                {
+                    column: (
+                        [rows[offset] for offset in offsets]
+                        if column == "a"
+                        else offsets
+                    )
+                    for column in columns
+                }
+            )
 
             request.send_response(200)
             request.send_header("Content-Type", "application/vnd.apache.arrow.file")
             request.end_headers()
             with pa.ipc.new_file(request.wfile, schema=table.schema) as writer:
-                writer.write_table(table)
+                writer.write_table(table, max_chunksize=2)
         else:
             request.send_response(404)
             request.end_headers()
 
     with mock_lancedb_connection(handler) as db:
-        permutation = Permutation.identity(db.open_table("test"))
+        table = db.open_table("test")
+        assert table.take_offsets([0, 2, 0, 4]).to_list() == [
+            {"a": 0},
+            {"a": 0},
+            {"a": 2},
+            {"a": 4},
+        ]
+
+        permutation = Permutation.identity(table)
         restored = pickle.loads(pickle.dumps(permutation))
-        assert restored.__getitems__([0, 2, 4]) == [{"a": 0}, {"a": 2}, {"a": 4}]
+        assert restored.__getitems__([0, 2, 0, 4]) == [
+            {"a": 0},
+            {"a": 2},
+            {"a": 0},
+            {"a": 4},
+        ]
 
 
 def test_create_table_exist_ok():