Deleting doubles

Somehow during a syncing process, my lists of contacts on my iMac, iPhone and iPad have become duplicated. How might this have occurred and how can I remove the contact duplicates? Doing it individually is tedious. Brinley Hughes Duplicates occur because of bugs in apps, format differences between Windows and OS X, iCloud communication errors and lots of other things besides. There are lots of commercial apps on both the OS X and iOS app stores that will remove duplicates. But you might not need one because the OS X Contacts app has a pretty decent duplicate-merging feature built in. Just click Card > Look for Duplicates…