Is it a good idea to put more film over an existing layer of tint?

Most professionals will tell you that it’s best to remove the existing tint before you start your work. With this approach, you can guarantee your work without having to worry about existing defects from someone else’s job. 

That being said, the customer is always right. If they insist that they want a new layer of tint over an existing layer of tint, do what makes them happy. If done correctly, there shouldn’t be too many issues.