fix(import): never overwrite an existing file when importing a folder

tests/test_import_flow.py

@@ -889,3 +889,42 @@ def test_imported_request_with_files_in_place_is_still_refused(client):
         "request_id": req_id,
     }]}).json()["results"][0]
     assert not res["ok"] and "bereits importiert" in res["error"]
+
+
+def test_two_titles_rendering_to_the_same_name_do_not_overwrite(client):
+    """Two books whose templates produce the same folder and file names must not
+    silently replace each other - that loses files and confuses ABS."""
+    root = client.tmp_path / "library" / "dcc"
+    lib_id = client.post("/api/libraries", json={
+        "name": "DCC", "media_type": "audiobook", "root_path": str(root),
+        "folder_template": "{Series}", "file_template": "{Title}",
+    }).json()["id"]
+    ids = [client.post("/api/requests", json={
+        "library_id": lib_id, "title": "Dungeon Crawler Carl",
+        "series": "Dungeon Crawler Carl", "volume": v,
+    }, params={"allow_duplicate": True}).json()["id"] for v in (1, 7)]
+
+    dests = []
+    for n, req_id in enumerate(ids, 1):
+        src = client.downloads / f"teil{n}"
+        src.mkdir()
+        for i in (1, 2):
+            (src / f"{i:02d}.mp3").write_bytes(f"Teil {n} Datei {i}".encode())
+        res = client.post("/api/import", json={"items": [{
+            "path": str(src), "is_dir": True,
+            "files": [str(f) for f in sorted(src.iterdir())], "request_id": req_id,
+        }]}).json()["results"][0]
+        dests.append(res)
+
+    assert dests[0]["ok"]
+    assert not dests[1]["ok"]
+    assert "already exists" in dests[1]["error"]
+    # the first title is untouched and the second one still has its files
+    folder = root / "Dungeon Crawler Carl"
+    assert sorted(p.name for p in folder.iterdir()) == [
+        "Dungeon Crawler Carl - Part 01.mp3", "Dungeon Crawler Carl - Part 02.mp3",
+    ]
+    content = (folder / "Dungeon Crawler Carl - Part 01.mp3").read_bytes()
+    assert b"Teil 1 Datei 1" in content and b"Teil 2" not in content
+    assert sorted(p.name for p in (client.downloads / "teil2").iterdir()) == \
+        ["01.mp3", "02.mp3"]