The last composed property/method/etc wins. In other words, you can overwrite everything when composing stamps.
In our practice the names never collide. Because stamps tend to stay very small thanks to its absolute freedom.
But if you still need to avoid name collision or protect a method from overwriting then use @stamp/collision utility stamp.
Maintaining ES6->ES5 is quite hard in such a specific project.
Most projects still transpile from ESNext to ES5.
We will rethink this policy as soon as there is a need which cannot be solved with ES5.