Answer a couple of questions and we'll show you how to fix it.
Make sure you're signed into the account you don't want to keep.
Open your profile and tap Settings.
Tap Delete Account and confirm. Once deleted, continue using the account you want to keep.
Find the player entry you want to remove and tap on their name to open their profile.
Tap the 3 dots menu, then Profile settings, then Delete user.
The player needs to find the entry without the green icon (the one you created) and open their profile.
This merges the entry you created with their registered account into a single player.
All scores and data from both entries are combined. The duplicate is removed automatically.
Tell them they have two registered accounts and need to delete the one they don't want.
They log into the account they want to remove, go to Profile → Settings → Delete Account, and confirm.
After deleting, they may need to rejoin the group using the invite code or link if their remaining account isn't already in the group.