When most people think of cloning, they picture biological cloning, where two organisms are given the same DNA. But to a physicist, cloning means something else: replicating an entire object down to the subatomic level.
If a physicist were to try and clone you, they would need to produce an exact copy, precisely replicating the position and momentum of every single particle, down to the smallest electron. Unfortunately, it turns out that due to the intricacies of quantum math, cloning is pretty much impossible. explains:
This explanation can get pretty technical, but in essence, the problem arises because in order to clone an object, you also have to clone all of the individual parts. The rules of quantum mechanics say that cloning the entire object all at once and cloning it piece by piece should produce the same result, but when you do the math that doesn't happen.
What this means is that cloning something is forbidden by the laws of quantum mechanics—but there is a loophole. If you get two identical copies of an object, you can create a cloning machine to produce more copies of that object. All you have to do is find the two identical objects in the first place. Simple, right?