mirror modifier duplicated my body I need to mirror the changes i am doing in the weights of my character from the left side to right side, but when i clic to add mirror modifier , it duplicates the whole body
With the Fast Carve Addon it is simple to use.

Download the addon here

Just weight paint one side. In the object mode -> Tool shelf -> Fast carve -> symmetrize. Click on the axis need. (If you have painted on X axis click on the X axis. It will symmetrize on the negative X axis).