How to unsalt roasted salted peanuts?
Depending on how much salt is on them, and how it's been applied, you might be able to knock some of it off, and effectively decant it:


Place the peanuts into a hard-sided container at least twice the volume of the peanuts that you can seal tightly.
Shake the peanuts. A lot. Not too hard, though, as the goal is to knock some of the salt off, not to smash the peanuts up.
Finish by shaking the peanuts side to side, or placing it on something that vibrates (like a washing machine)
Open the container, and scoop the peanuts out, being careful to leave the excess salt at the bottom.


You save yourself some effort, you might be able to do the whole work on top of your washing machine, or leave the container in the trunk of your car for a few trips.  The larger the salt crystals, the easier this will be to remove.  If it was applied as a wet spray or while the oils were on the surface of the peanuts, it's going to bind more to the peanuts, and be more difficult to remove.

Although the diluting trick will work, you can also use the peanuts in a recipe that would have otherwise called for salt and leave out the additional salt.  You could also crush them up and use them as a topping for something that could use a little extra salt.  (ice cream, Pad Thai, etc.)