Nonty's Uncle Leo created an evil clone that made clones of Monty. One of them did bad things at Monty's school so he would get blamed for them. They trapped Monty in Leo's lab and planned to induct him into their clone group.
Nan got helped and managed to round all the clones up, along with Monty who was mistaken for a clone while one of the actual clones lived his life. The real Monty escaped but the clone Monty tricked them again and Monty was sent away again. Clone Monty revealed this to Nan and it is unknown if the real Monty was ever rescued.
Because they are stronger and don't feel pain, they believe they are superior to the real Monty. Monty used this to trick them into helping him escape the truck, making think they would get their revenge.
The first clone Monty saw is especially evil, as he orchestrated many of the evil acts that Monty was blamed for. He also proved to be intelligent when he switched doughnuts full of peanut out with regular ones and pretend to be allergic so that he would be mistaken for the real Monty.