Mask help

I have a password hash I'm trying to crack. There is a pattern with previous found passwords in the group in that they all repeat 1 character once, and they're all the same length. How can I create a mask to brute force all passwords of the set length while making sure each generated guess repeats only 1 of the characters once? For example, I'd like it to guess:
abcd12345a
abcd12345b
...
abcd123455

but NOT guess:
abcd123456 (all characters unique)
abcd1234aa (1 character repeated more than twice)
abcd1234ab (2 characters repeated twice)