I can't get one of my two imap'd Google Apps accounts to work in Mail.

When I try to save the gmail address in Preferences > Accounts > Account Information > User Name, I get the following message:-

The account path /Users/dom/Library/Mail/IMAP-dom@ltrow.com@imap.gmail.com is already being used by the account “Gmail1”
Gmail1 is the other Google Apps account and it works fine. It looks like Gmail1 is using a folder that dom@ltrow.com needs to use. Why, I don't know.

I have no idea what this means beyond believing that it is referring to a folder/file in my Mac's Library.

It's possible I have created a problem further back by messing about with Mail while trying to find the ideal solution to sync my iPhone, Mac and Gmail accounts. However, at present, the only two accounts I have in Mail are the imap accounts Gmail1 and Gmail2.

Can anyone help me to get the dom@ltrow.com address to work?