Adding a model ID that was already on the list in the connection settings modal simply appended it again, so the same model could sit in the whitelist any number of times. The arena model modal had the same flaw, its dropdown kept offering models that were already selected.
The connection modal now rejects a duplicate with a toast and trims the input first; surrounding whitespace renders invisibly in the list, so an untrimmed ID would slip past the duplicate check and still show up as a visually identical row. The arena modal instead filters already-added models out of the dropdown, matching the existing model selector in the admin settings, so a duplicate can no longer be picked at all. Both modals also drop duplicates when loading a stored list, so configs that already contain them are cleaned on their next save.
Until such a config is re-saved, one residual effect of old data remains: a duplicated ID in an arena model's stored list keeps double weight in the random model draw. New duplicates can no longer be created through the UI.
Fixes#28249