NT 4.0 Domain Trust Problem

We have 3 domains that require complete trust (ie 2way). the domains are A, B and C. The trust is working fine between A and B, and also between B and C.

However when trying to establish the trust between A and C we can add domain C to trusting domains from DC A, and we can add domain A to the trusted domains from DC C.

But when we try and add domain A to the trusting domains from DC C we get the following error message “The user account already exists.”

I have removed the trust from both sides and recreated it but this message still appears.

Any Ideas?