Wow I have never heard of dewclaws on the rear legs, let alone double ones! I would def remove the floppy one during neuter but leave the other one IF the vet says it is securely attached. Normal securely attached dewclaws under normal circumstances should not hinder their ability to do anything.
One of our members here Mimimomo had one of her yorkies rip out a dew claw on the carpet. Ouch! He had a floppy one I guess. That is the only worrisome thing. If they are secure then no worries, other than you have to check it for nail length often since it does not touch the ground, and therefore walks will not file it down naturally.
__________________  The T.U.B. Pack! Toto, Uni, & Bindi
RIP Lord Scrappington Montgomery McLimpybottom aka El Lenguo the Handicapped Ninja 10-12-12 
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