Two identical iCloud Group lists in Contacts
Found it!

I have no idea how it actually was set up, as I discovered a pref pane through another question on Ask Different which I'd never seen before…

System Prefs > Users &amp; Groups >[unlock] > Right click user name > Advanced Options...

My user@me.com account was listed twice in aliases.

I deleted one - the duplicate iCloud account in Contacts disappeared.
So far I've seen no unwanted  side-effects.