That’s awesome, but for some people, the problem isn’t duplicated info—it’s extraneous accounts clogging up the program. To check yours, open Contacts, and if you don’t see the sidebar shown below, press Command-1 (View> Show Groups).
I’m currently allowing four accounts (in yellow) to sync contacts to my Mac. If you actually need to sync data from different sources—for example, both your personal contacts from iCloud and your email contacts from Google—that’s all well and good, but for most folks, having just one list sync consistently across their devices is ideal and leads to the least amount of confusion. I can’t tell you the number of people I’ve talked to who were “missing” contacts, only to find that their Mac was syncing by default to a Google account that wasn’t even configured on their iPhones. Ick.
So how do you fix this? Well, first you should start by verifying that there aren’t any contacts you need to copy from the accounts you’re gonna remove before you do so. (I’m assuming here that the iCloud one is what you’ll leave standing, but you do what makes you happy.) Within Contacts’ “Groups” sidebar shown above, you can click on any of the “All” lists to see only the contacts associated with that particular account. Then just drag any contacts that don’t exist within your chosen account over to it.
As you can see, I found Mr. Testerson’s card within my junk account, and I dragged him on over into iCloud. You’ll want to repeat this process until you’re pretty sure that whichever account you’re leaving on has everything you’ll need. Of course, if you’re not a micromanager like me, you could just select everything within an account and move it all over to the one you’re leaving on, but I’d strongly suggest you make a backup of your contacts first if you go that route (File > Export > Contacts Archive).
Anyway, after getting everything combined into one list, you’ll head on over to System Preferences > Internet Accounts.
Within that pane’s left-hand list, you can again tell which accounts have contact syncing turned on.
Go through and click on each account you want to turn off, and then you’ll just uncheck the appropriate box, leaving on the one account that you’d like to sync.
Note that turning off syncing for a particular account doesn’t actually delete those contacts from where they live on the server, so you can always toggle the syncing back on if you find you’re missing something later.
Whew! Now doesn’t my contacts list look nicer?
After your Mac is all cleaned up, go and check your iOS devices, too. Similarly, you can see which accounts are being allowed to sync contacts within Settings > Mail, Contacts, Calendars.
If you toggle off contacts syncing for all except that one default account you chose on your Mac, everything will be all nice and neat. No more worrying that you’ll be sending contacts to that obscure Gmail account that you use for junk mail! Hey, that may sound like an odd problem to have, but I see it all of the time, people.