rules file support in attack mode 3

I can use command like this to bruteforce hash

But how can I let the hashcat know that the first four ?d?d?d?d are the  same as the last four ?d?d?d?d (For example, password like: 12341234,23452345)

Is there anyone know how  to modify the hashcat source  to support new masks like: ?d?d?d?d -r d ,which means ?d?d?d?d?d?d?d?d but the first four ?d?d?d?d are always the same as the last four ?d?d?d?d.(It is something like a Rule-based Attack,but I want the rule to support bruteforce attack,attack mode 3) Is it easy or not? And which part of the code should we focus on?