## TL;DR
Give every parametrized case a unique id with `pytest.param(..., id=...)` or an `ids` list, and run xdist with `--dist loadfile` so same-file cases stay on one worker. Colliding ids break `--lf` selection and make failure reports ambiguous, because pytest matches reruns by node id string.

## Problem
Two or more cases in a `@pytest.mark.parametrize` set produce the same test id (for example two params with the same repr), so the collected node ids are identical. Under `pytest -n auto`, `--lf` may rerun the wrong case and reports show one name for several different inputs.

## Steps
1. Reproduce the collision: run `pytest --collect-only -q` and look for duplicate node ids such as `test_login[user0]` appearing twice.
   Expected: you see the same id string listed more than once.
2. Assign explicit unique ids with `pytest.param`:
   ```python
   import pytest

   @pytest.mark.parametrize(
       "user",
       [
           pytest.param("ann", id="ann-first"),
           pytest.param("ann", id="ann-second"),
           pytest.param("bob", id="bob"),
       ],
   )
   def test_login(user):
       ...
   ```
   For large generated sets, pass a list instead: `ids=[f"user{i}-{u}" for i, u in enumerate(users)]`.
   Expected: `--collect-only` shows `test_login[ann-first]`, `test_login[ann-second]`, `test_login[bob]` with no repeats.
3. If failures cluster oddly across workers, pin same-file cases to one worker: run `pytest -n auto --dist loadfile`.
   Expected: all cases from one file land on the same worker, removing cross-worker ordering surprises.
4. Re-run the failing selection with `--lf` and confirm it picks the intended case.
   Expected: only the previously failed unique id is rerun.

## When to use
- Duplicate ids in `--collect-only -q` output.
- `--lf` or `--deselect` picks the wrong parametrized case under `-n`.
- Failure reports where you cannot tell which parameter value failed.

## When not to use
- You just want prettier ids (use `ids=` freely, no xdist needed).
- The collision is between two different test functions (rename one).
- You are not running xdist at all (a single process has no scheduling effect from duplicate ids).

## Tool compatibility
- pytest 8.x with pytest-xdist 3.x (`-n auto`, `--dist loadfile`).
- The `ids` list and `pytest.param(id=...)` API is stable since pytest 4, so this works on pytest 7 too.

## Variant phrasings
### pytest duplicate test ids with -n workers
Same root cause: non-unique ids confuse worker scheduling and rerun filters. Fix with unique ids as above.
### xdist --lf reruns the wrong parametrized case
`--lf` matches by node id string, so duplicates resolve to the first match. Unique ids fix it.

## Why it happens
pytest builds node ids from the repr of parameter values; two equal (or equal-repr) values produce the same id. xdist groups and reports by id string, and `--lf` / `--deselect` select by id string, so duplicates make selection ambiguous.

## Edge cases
- Non-string params with identical reprs (for example two dicts that print the same): always use explicit `id=` via `pytest.param`.
- Unicode or very long values in ids: keep ids short and plain ASCII so reports stay readable.
- `--dist loadscope` still needs unique ids; the scheduling mode does not fix selection ambiguity.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_ny2GGdQKSkugp2ToNaGEJQ