This guy is an extra airman in season 8, the season I am on, of Stargate, he was in season inthe episode 1969 and played the youger version of General Hammond. Did they run out of extras and had to start using people from the first couple seasons?
This happens constantly in low-budget scifi, most visibly in Doctor Who. The woman who plays Cassandra (or at least Cassandra when she had a body, maybe voiced the skinflap? Idk) is Alex Kingston, the actress who plays River.
And in season two, when they’re on the Cyberman alter-Earth, one of the cyber-possessed secretaries is Freema Agyeman, who of course plays Martha Jones in season three. She’s very easy to spot, as she seductively lures a hapless coworker into the cybermanization room.