Linked iCloud and iMac Contacts are duplicating in Mail

We have 2 iMac computers & 1 iPhone. All three use Contacts connected to iCloud with iCloud being the preferred. On my Mojave computer I am trying to send our emails to groups under bcc in Mail. The email addresses are duplicating in no exact order in the bcc. I am limited by my website service to a certain number of emails per hour to prevent spam warnings. I found out when I quickly went over my limit. Now I have to click to expand group and delete the duplicates. I have done the "Look for duplicates" on both computers and in the iCloud Contacts. I did notice I had a couple of duplicates in one of my groups but the Look for duplicates did not find them to merge. I found that I could drag one to either the All iCloud label or All on My Mac label (whichever one would take it) and they would combine. That's a whole other problem. I would like to know how to fix both with the duplicates being top priority. How can I stop this from happening?