How to fix it? Stop setting opposing values in two different scripts? For example, if you had a variable called “score” and you set it to “500” and in another script you set it to “1000” then there is no fix, it’s just bad code with a bug setting the same thing with conflicting values i.e this isn’t specific to velocity.
This is why you don’t normally set the velocity directly as if you do this in multiple places, you’re just stomping over yourself. You can use forces or just design it so that you’re not setting the velocity directly in multiple places.
If you haven’t already, I would highly suggest you follow a bunch of tutorials on how to deal with movement using Physics, there are plenty around.