Having trouble adding teammate popup

Question

I've received an email requesting access to my Trello board.
Our board has 2 admins and we both tried to grant access but received this popup:

However, we can see that it looks like the user is actually a member of the board. 
Does anyone know why this popup is showing and what it means?

Brittany Joiner · Answer

@Lucy Hill strange indeed!

Lucy Hill · Answer

@Brittany Joiner We haven't seen this with any of the other guests on my board and none are associated with my domain, I invited them all using the link provided (I'm on the free Trello account and sharing this board with stakeholders for a company I'm working freelance for if that makes a difference?)
I've contacted the user and they do have access to the board, so I'm wondering if this popup should say they're already a member, rather than they can't be added?

Brittany Joiner · Answer

@Lucy Hill are they logged into another trello account? Perhaps they're trying to access with a personal account instead of an account associated with your company's domain?