One of the most common complaints I get from clients is that their contacts list is all messed up. Maybe you've got 19 copies of John Smith's contact card, and heck, you don't even like John. Or maybe yours aren't syncing between your devices. It's a frustrating problem, and trying to clean it up can be something of a nightmare.
How you'll fix this depends on the cause, of course. If it's that you're using several different services to sync stuff (for example, if you have a set of Google contacts and then a set of iCloud ones), you may have to try to consolidate things into one list, which can be an involved process. But if you're just seeing several different cards for the same person as I mentioned above, your Mac has a built-in way to merge those duplicated contacts. This'll keep you from having to wade through all of John's cards to find the info you've got on him.
To do this, open the Contacts app on your Mac, and then select all of the cards you'd like to merge. The easiest way to do this is to click on the first one, hold down the Command key on your keyboard, and then click on the subsequent cards, which will select each one like so:
See how the four contacts with "test" in them are now selected? I did that by holding down Command while I clicked them. Anyhow, after your affected cards are highlighted, you'll go up to the menus at the top and choose Card > Merge Selected Cards.
Do that, and you'll no longer see all of those duplicates of ol' John; the Contacts program will take all of the cards you selected and put their data into one. At that point, it'll be way easier to edit the information to reflect the person's current info. I mean, this is all assuming you want to keep any of John's cards. I hear he's a real pain.
Finally, you may notice that there's a different option under that same "Card" menu: "Look for Duplicates." I don't generally do this with my clients or recommend it just because it's somewhat automated, and I think it's safer to clean things up manually. But that's just me, and I'm paranoid about my contact data. Well, to be fair, I'm paranoid about everything computer-related. That's why I constantly harp on making backups in almost all of these articles! Even in unrelated posts like this one.
Oh, yeah. Speaking of which, have you checked your backup today?