THEORY
So I have a possible new piece of evidence that supports clone shiro that I have not yet seen others bring up. And it could be a reach but this scene is what sold me on the clone theory.
So in season 3 episode 6 during the fight with Lotors crew, Keith has to decide either to go after the comet ship thing or the cargo ship. As Voltron is getting hit with heavy fire using its shield to protect itself; Keith and Shiro are discussing which to attack and then it cuts back to Lotor communicating with the ship attacking Voltron.
Lotor says “We have them right where we want them. They can either defend or go after the teledav and leave themselves vulnerable.” This tells us he doesn’t really know for sure which choice they are going to make.
Then it shows Shiro talking to Keith telling him “You’re going to have to lower your shield, shoot the cargo ship, and deal with the consequences.” After that Keith (still communicating with Shiro) tells hunk to get ready for when they lower their shield that their going to attack the cargo ship.
IMMEDIATELY after that it cuts to Lotor saying “They’re going to attack the cargo ship” how else would he know for sure? Last he knew it was an either or situation yet now he’s pretty darn sure what their move is gonna be. And voltron didn’t show any clue what their next move was. UNLESS clone shiro is spying on them feeding Lotor information (unknowingly?). BAM.