The question is a bit vague, but near as I can tell, you are receiving mail from "firstname.lastname@example.org" to "email@example.com" and what to carbon-copy all inbound email to "firstname.lastname@example.org".
Easiest solution is to put an alias in front of the receiving address, then specify both the local address and the remote address in the alias. The entry will look something like this (but not exactly):
localuser: localuser, email@example.com
Place the entry in the /etc/aliases file, then run