Rule sets / OBR / Double Set Balk
Double Set Balk
A pitcher may not come to a complete stop in the set position, restart his motion to come set and come to a second complete stop.
If there is a runner, or runners, it is a balk when the pitcher delivers the pitch from Set Position without coming to a stop.
MLB Umpire Development Manual Authoritative interpretation
Lastly, if the pitcher comes to a complete stop, starts his motion, then makes a second stop, he shall be charged with a balk. A pitcher is not allowed to set twice. This is referred to as a "double set."
Video examples
Left-handed pitcher Wade Miley comes to a complete stop in the Set Position, starts his motion, then makes a second stop.
Right-handed pitcher Johnny Cueto comes to a complete stop in the Set Position, starts his motion, then makes a second stop.