Anything with exception of Unique items can drop only once in same game session, period. Yes you can see more unique rings, amulets, facets… because they are all different unique items as rings and facet are not same.
This is simply a fact and anyone who says otherwise is either wrong, mistaken or simply trolling.
This is also easily testable:
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with modding where you set quill rat to drop only unique rings. You will see that they will drop every unique ring only once in same game.
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By playing and checking items and eventualy you might be lucky for example when you see mephisto dropping 2 shakos and one is unique, it might happen that second one rolled unique too but since it can drop only once, it will have trille durability.
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Easily testable offline when you summon Diablo clone twice in same game, fist one drops annihilus, second one will only drop magic small charm because yes, rules do apply also to unique charms.