What most likely happened is when you where painting the one side you hit some of the verts on the opposite side... even though you where all black you add a bit of weight on the wrong side....
You can clean this up by going into the component editor and select the verts that have the weght on and change then too zero.
Hope that helps
K
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb