This whole video just shows an exploit when you can use an enemy teleporter as a disguised spy and quickly change class, making it gamebraking in no respawn time servers. But this exploit was patched 14 hours later after this video was published 3 years ago so theres nothing to worry about.
"Fixed teleporters teleporting players that respawn or change team during the teleport animation. This fixes an exploit possible when playing on some "instant respawn" community servers"
Not sure how this even impacts duel. Players can already go to the enemy spawn by walking around the rafters on the map for a while and even if this exploit existed which it no longer does, nobody can get a teleporter up without it being killed and even if they did, you can walk to the enemies spawn to shoot at them literally within 5 seconds from your own spawn. AND EVEN IN SPITE OF THAT, building a teleporter into your own spawn for enemy spies is already against the rules.