Weight painting on low poly model acts weird i've been trying to weight paint this simple model for a while now because the automatic weights didnt work that well. The images below indicate the places where it went wrong, even though i have duplicated the arms and legs and the opposite limbs work perfectly, those limbs are completely red. As you can see when i weight paint i cant select some vertices without fading into opposite one. If you dont understand my point, heres the link for the blend file: 

https://drive.google.com/open?id=11rx-Kgh_FhNLNz8_GN3mrIYPPevsmrno

below are the pictures
You have duplicate vertices in your model, and you are working in opaque mode ('limit selection to visible'), so only one of the duplicates can be selected. Several ways to get around this, one would be to select the vertives you want to work on in edit mode, then switch to weight paint and activate the icon directly to the left of the layer selection block (a cube with one highlighted vertex, "vertex selection masking"). Then you'll be able to only work on the selected vertices (you might need to unselect 'limit selection to visible').

Or you could remove the doubles in edit mode...