It's mentioned in the Allies section of the rules. You would now have access to the Elite Rebel Saboteurs.
Wait a sec...I'm 99.9% sure that's not exactly right because they're both green missions...you can't have 2x green mission that awards the same ally iirc
Target of Opportunity
Armed and Operational
RRG pg. 17 wrote:
Four green Side Mission cards chosen by the hero players.
Heroes cannot choose more than one of these cards that
rewards the same ally, regardless of the ally’s subtitle, such as
“Hero of the Rebellion.”
I think the answer to OP's question should be "The Imperial player messed up the side mission deck, this is illegal"
My old post