Trouble adding a MS EXchange shared mailbox to Mail

I get the following message when trying to add a shared mailbox from my MS Exchange server, any ideas?

INVALID DIRECTORY

"The account path /Users/Kieren/Library/Mail/V2/EWS-411924@vmail.virgin-atlantic.com is already being used by the account “Kieren.J.Blake”

Is there a way of adding a shared mailbox from MS Exchange into MAC Mail?

Many thanks

Kieren