Armature X-axis mirror not weighting correctly I have been rigging a model and have come across an issue that I haven't been able to solve after a lot of experimentation, basically, my current model is mirrored with the mirror modifier, and I am rigging it with X-axis mirror on. 

Normally, when I weight paint say, the left leg to bone 'Leg.L', then it automatically weights the other leg (which is the mirrored side with the mirror modifier) to 'Leg.R' automatically, and it is doing that here for the most part.

The problem is, for some reason my 'Hand.R' and 'Hand.L' bones are not behaving in this way. When I try and weight the hand to 'Hand.L' it weights the vertices on both the left and right hand due to the mirror modifier on the model, rather than mirror only the left hand I am working on to 'Hand.L' and automatically apply the weights on the other side to 'Hand.R'.

I am fairly unfamiliar with rigging in blender using mirror, so I am not sure if there is a setting somewhere that I am missing, or why this part of the armature is suddenly not mirroring fine like the rest of the rig is. Any help / explanation would be greatly appreciated.

Apologies if this is a duplicate, I wasn't quite sure what the proper wording would be, I hope my explanation of the issue is clear enough.
This issue is, as you have identified, due to the mirror modifier. Technically speaking there is only one hand in the geometry data and the weight painting system is only reading one hand in two places, because weighting is technically a vertex group assignment system. If you apply the mirror modifier (I suggest making a duplicate of the hand if you aren't sure about needing to make changes later.) it will create the geometry for the mirrored hand. Then you can go back into weight painting and the x axis mirror should work as intended.