Get Rid of Dialog box

Two days ago, I started getting an annoying pop-up saying "[my @mac.com email address] is already signed in. You can keep using the already logged in iCloud account or use another one". Aside from being a poorly constructed message, the second sentence is unintelligible. The two choices to respond are "Use another account" and "Use '[my @mac.com email address]'". What other account? Use '[my @mac.com email address]' for what? This ranks right up there for useless instructions written by an obviously English - challenged software engineer, which keeps coming back about 50 times a day. Can anyone tell me: (1) why is this popping up? (2) what does it mean? (3) how do I bury it so I never see it again? Thank you.