How to remap a single key?

I have a broken key on my Mac's keyboard and I need to remap it to some other key, by double press. Temporarily. It's not a special but an alphabet letter one. I want to remap it this way: "f" (broken) --> "double a" How can one do it? Preferably without any third-party tool. **update1** This hasn't worked - double a won't get replaced with f whenever I type "aa" anywhere on my laptop. I want it to replace double a with f on the fly.