Thanks for your reply.
I meant with duplicates to the same email AND the same name. I think it's the approach most of the applications take. I use MailMate and duplicates don't show. It's clear that if you aren't very carefully, some duplicates can't be filtered. But at least, it's up to the user to maintain its contacts list clean. Also, from a usability point of view, it's weird that a selection list is populated with exactly the same entry several times.
Do you think it could be interesting to implement this approach?
Thanks!
PD: I don't disable my Google Contacts account, because I would like to keep my contacts synchronized. It's a pity that groups don't synchronize and I have to do it manually, causing problems like this issue.