It's probably as simple as someone else owns the @google acct that is equal to your Yahoo account (ie like Billybrown@yahoo might be taken in Google and not available. I might've misread a bit so if that sounds too easy etc my bad but the fix would be to add a number or something else To ensure its not taken by someone.

I actually own both accounts with the same user name and set up both. the gmail one gives me difficulty by asking me to login to my yahoo as primary. in yahoo I cannot add my gmail account because it tells me it is already taken! don't know how to combine both into one account with say gmail as the source.

should I delete both accounts and set up the gmail account again as new?