On prior version of Anki, when I added a new card that was a duplicate of prior cards, it would highlight the field to alert me (I think in red). I noticed today after upgrading to the newest version of Anki 2.1.29 that this was no longer happening.
I attempted to downgrade, but this did not fix the problem of Anki checking for duplicates.
When Anki checks for duplicates, it only compares other notes of the same type. Thus if you add a capital city called “Orange” using the capital city note type, you won’t see a duplicate message when it comes time to learn how to say “orange” in French.
to clarify more and in a simpler way, Anki won’t detect a card created from Basic (and reversed card) model and a card created from Basic model as duplicates even if the front field for both of them is the same text
also if you have a Basic 1 and Basic 2, even if those note types are identical, if you create the same card in both of these note types, Anki still won’t recognize them as duplicates.
the cards should use the same note type (both Basic or both Basic 2) in order for anki to recognize them as duplicates.
also, for cards that are created in the same note type, Anki will detect duplicates in different decks