why are contacts being added to my groups without me doing anything

Hi I have set up a couple of groups on my Mac, but if reasons unknown to me occasionally a random contact appears in the group. I thought that had to physically add them to the groups. Some contact seem to duplicate between the groups and occasionally when I create a new contact they appear to be added automatically to the group. I send emails using my groups from 2 Macs OS Catalina 10.15.4 and a iPhone. OS 13.5