You need to duplicate the source compositions, not the text layers in the timeline. A pre-comp can be used a million times across different layers/ comps, yet it is still just one reference to the original. inevitably it will change whenever you edit it, regardless from which place in the project. You might want to read the AE online help on the basics. No offense, but this is one of those classic newbie traps and only educating yourself will avoid struggling with such issues.