Add warning to run_model_on_task with avoid duplicates if no authentication

[PGijsbers]

see #1145

[Prachi194agrawal]

Hi @PGijsbers,

I noticed that this issue is still open and the previous linked PRs (#1240 and #1246) seem to be inactive since early 2023.

I would like to pick this up. I plan to add the UserWarning in openml/runs/functions.py when avoid_duplicate_runs=True is passed without an authenticated API key.

I can submit a PR for this shortly!