* In case a subset of IDs is given, it was not checked that all items
are ID data.
* In case of error while processing the subset, there could be some
python data 'leak' (the `user_map` would not be unreferenced).
* Improve performances in case there are many duplicates in given subset
iterable of IDs, by not processing the duplicates.
Discovered in !127252 review.
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