Photo IDs

In the Settings app, select the top item in the left column (showing an icon/picture, a name, and "Apple ID"). That should bring up the Apple ID pane.

At the top of that pane, you should see a larger version of the same icon or picture (which I think is what you are asking about), the same name, and an email address that identifies the Apple ID.

Do you see the same email address when you do the above on both user accounts? If so, then they are sharing the same Apple ID. Recent versions of the system will want them to have the same icon/picture.

Sounds like you have two user accounts on the iMac, but is it possible they are connected to the same Apple ID in Settings?

As long as you have the one different character typed correctly in each case, the system should not get you confused. My wife and I have a similar arrangement, except that our Apple IDs differ by several characters. We don't have the problem you describe.

I think your problem goes beyond what we volunteers here in the Support Communities can handle. You can consult the professionals at Apple Support proper by clicking here: Official Apple Support