Looks like you’ve got vertices weighted for bones in both boots associated with each other. Remove left-side vertex groups from the right boot and vice versa. If that doesn’t work, see below.
Here is what I would do (simplified version):
Separate the boots from the legs if possible.
Delete one boot (say, the right one).
Cleanup any holes or whatnot from the remaining boot.
Add a mirror modifier, making sure the center of the boot is at the origin.
Make sure the mirrored boots don’t intersect – you can adjust the mirror modifier settings to prevent this.
Make sure there are no right boot vertex groups associated with the left boot. You can do this by moving the right foot/leg and checking to see if any verts move on the left side. If so, remove them from the right side vertex groups.
Apply the mirror modifier. Check to see if the vertex groups mirrored over. If not, then you’ll have to assign them.
Seems like a long drawn out way of doing it, but it goes pretty fast once you know what’s going on.