Technically its the duplication of the TO: line.
With the caveat that I have not looked at the code...
The protocol requires that each recipient be identified individually. When you send an email with 3 people in the "to:" the server actually breaks that down into 3 "RCPT TO:
X@X.com" commands that are sent to the server on the other end of the communication... assuming the code it is going to acheive the same thing.