You could use the same UV guides (no real need to change it) but make a different texture map for left and right shoe (Making note to keep the position of the decal the same, but mirroring it), and then make sure they have different mats assigned to left and right shoe such that they use the left and right textures. This is probably a lot easier than re-doing the UVs on a model provided it's not too bad in most other regards. (UV mapping can be a real PITA sometimes, and that's even when you have good tools for the job and understand the process.)

I suspect the person making the model expected maps to have textures and color, but not decals - so they cut corners by using a mirror of one model and sharing the same UVs on both. Considering how they assumed the model to be used 99%, this is probably an understandable shortcut too. (And not forgetting the mention in above paragraph.)

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.