Or alternatively, the odds of a particular mRNA mutation doing something harmful are astronomically low. With 7 billion people being injected with large amounts of mRNA, you have a very small number times a very big number, so I can't estimate how likely something is to happen, but we know it's small since we haven't learned about anything critical with tens (hundreds) of millions of injections so far.
If an mRNA mutation were to make a self-replicating virus, or make something which damages DNA leading to cancer, or something wonky like that, I guess that's in abstract possible. It can make abstract proteins, and we don't know what those will do.
In that case, if I have twice as much mutated mRNA in my body, the odds of that happening almost exactly double. It's like rolling a 1-trillion sided die twice instead of once and seeing if I roll a 1. With one roll, I have a 1-in-a-trillion chance. With two rolls, I have a 2-in-a-trillion chance (minus 1-in-a-septillion of having the same thing happen twice, which we don't know what it would do, but is a small enough possibility we can ignore).
But yeah, unless something really wonky going on, I'm not going to end up with hundreds of strands of mutated mRNA in my body *all doing the same thing*.