Very well said and yes its true, the pre comps within my main comp that I am wanting to duplicate are altered on both comps and you have just explained why. Its great to now understand how this all works, Im still struggling with this though and I will explain why
So as you see in this image I have a picture that is animated onto the screen with a watercolour effectw hich consists of many pre comps..

all i am trying to do is add an other image but on the left hand side and have it come onto the screen in the same animation, but to appear later on, when I try to duplicate the comp I cant have both pictures on the same screen as they are separate comps. I then tried to duplicate the specific pre comps but the issue is, is that the effect is over on part of the screen and to then duplicate the pre comps doesn’t work in this instance as I need the watercolour effect to happen twice at different times however it cannot, it happens once and then doesn’t work when you put the duplicated effect over the original if you understand? There is so many layers to trying to explain this its a headache to try and word it hahaha Im sorry for my awful explaining man