I have seen some similar behavior on SP4 and in our case it was due to adding the same account once using the FQDN and once using the short name (see example below).
Usually, you should not have any need to have multiple accounts specified if you always use the same names - again, see example.
Example:
Task Join domain:
domain info entered: mydomain
or domain info entered: mydomain.something.more.com
For such scenarios you will need to add the short domain account first and then the FQDN account.
-BBC