Snake touched on a couple things that couse doubling in an SKS:
Dirt or preservative compound in the firing pin bore in the bolt. (It should be clean and free of any cosmo, oil, etc.)
The firing pin is installed incorrectly. (Possible, but not likely to cause doubling, but usually causes FA-empty of the entire mag. There are four or five different designs of SKS firing pin, and one specific type found in Chinese SKSs can be installed backwards, which when so installed will have the FP tip ALWAYS protruding through the boltface. Can you say "Full Auto?"
A worn disconnector or sear face in the trigger group. (I have only seen this on one SKS owned by a friend's acquaintance; he took the recommendation to install a new trigger group for $35)
"Floating" the trigger finger. (In some cases, individuals will not follow-through with pulling the trigger all the way to the rear stop. If you just pull it enough, the weapon will fire and the recoil will tap the trigger against your "floating" finger. The result is a second or even third shot. When done intentionally while holding a semi-auto but the front handguard with the weapon at the hip and a holding the trigger finger stiff, it is called "bumping" or "bump firing". Lots of semi-auto owners like to rip off a mag in this pseudo-full auto manner.
If your firing pin and pin bore in the bolt are clean and free of any preserv or lubricant (don't oil the FP, it will attract dirt), it is most likely a trigger finger float or a worn disconnector. Clean the FP and its bore first by tapping the rectangular-headed retainer pin out and removing the FP from the rear of the bolt. Mind the orientation of the flats and grooves on the FP as you remove it so that is can be reinstalled. If you drop it, look for the burnish marks on one of the pin flats that were made by contact with the FP retaining pin.
HTH
Noah