There have been multiple complaints like yours over the years; to my knowledge, none has ever been verified to be a Quicken problem.
[I have tested many such complaints in the Quicken versions named by a poster, including yours - and my Address Book imports have succeeded every time.]
The fact that you found that the addresses you wanted imported were in Quicken multiple times, guarantees that your multiple attempts to import succeeded.
One user mistake that has frequently occurred, is failing to look in the <unassigned> group for the imported addresses. Users tend to think that they can import addresses to an Address Book Group of their choice: but that is not possible.
ALL imported addresses will be imported to the <unassigned> group - no matter what group the user attempts to specify.
Funny thing. I log on tonight and see my addresses uploaded 5 times. Never were there earlier when I tried several times using different combinations of CSV and TXT and ensuring the field names matched etc. Anyway I just spent 50 minutes deleting the ones I don't need.