CustomCollectionModelBinder's `as IList` cast does not work for HashSet,
breaking the code around trailing []s in query arguments.
The values are still being treated as a set in the query so adding the
same value multiple times shouldn't do anything except hit the max limit
quicker.
The intention was to aid clients which may not be doing their own
deduplication but I haven't really observed that behavior in the wild
and I doubt it's anything too much to expect from clients.
That said, testing on mastodon.social shows Mastodon itself may be doing
deduplication here before checking the limit, though I'm not entirely sure
if this will ever be noticed in the wild.
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