select all vertices, pick Modify\Submesh\Weld. Expand it's options.
Enable "Multy target mode" there;
set up angle precision (smaller value = less smoothness; bigger value = more smoothness);
set up distance precision (a pretty small value will fit your needs here)
set up uv distance precision (1.0 is ok here).
click in viewport to weld vertices in specified percision limits; If you unsatisfied, you can press undo, change settings and apply again.