Two sets of opposing nails per toe???

We have a cat who has regular nails at the top of his toes on his front
paws, and then a second set of nails going in the opposing direction on the
bottom of his toes (curling up from the pad). The bottom set is dark brown
and has no discernable point to indicate where they should be cut. The top
nails are regular.

Has anyone ever seen this before? Do you know how to cut nails like this? I
would hate to have to take him to the vet every time he needs a trim, but
his nails are getting really long on the bottom and it's apparent they need
to be trimmed.

Thanks in advance,

Marjorie