Remove or merge double contacts

See -> If you see duplicate contacts after setting up iCloud Contacts

If you see duplicate contacts after setting up iCloud Contacts - Apple Support

This link doesn't help - after doing what is written in your link - 27 double contacts are found as double, but I still have 3200 and each of them double!

There must be a solution to do this in one move..

I don't understand that apple can't fix this with one update and this would not appear every time you buy new Mac..