Mask + Brute Force with repeated password

Hey everyone,

New to the forums here, and have found a bunch of great information so far!  I am just getting familiar with using masks and customs rules, and haven't been able to find an exact answer to my question:

I know the beginning and end of my password, but want to brute force the middle portion.  However for every brute force guess, I want to duplicate the characters in the middle because I know I repeated my "subpassword" twice.  For example:

[know this part of the password] +  bruteforce + bruteforce + [know this part of the password]

I would like to increment up to a character length of 8 (for a total of 16 in the "subpassword")

so one guess might be: [known] + pass123pass123 + [known]

Thanks in advance for any help, and hopefully this makes sense!