Problems while exporting/importing and integrating the scenes

Hi there, when working on some project, we are 4(ABCD) ppl as a team, one project is divided into 4 scenes and all the 4 scenes contains one same 3D file(lets name it as character).

A will work on scene 1, B will work on scene 2, C will work on scene 3, D will work on scene 4, everyone will work in different different systems and, they will do different different changes in the 3D file (Character).

all the ppl (ABCD) has same materials for Character . But they have tweaked all the materials differently as they wish.. now, i want to create a new scene called Integrated Scene in a new project to integrate all the ABCD s' files... now, what happens is, as all of them have used same materials and same 3D files, when i import each and every scene, all the lightmaps, materials are getting spoiled and 3D files are getting replaced.

so, tried renaming the folder names and 3D files before exporting , and when i import that scene package, even if the 3D file name is changed, it is still reading the old file and replacing the existing the 3D file,

so, to avoid this, what can be done? i want all ABCD s' scenes into one single project and all the 4 scenes should be integrated into one single scene...