There can be a problem in the trigger group that will cause this. I copied the following info for safety from:
“Survivors” SKS FAQ
December 31, 2003
©2003 L. E. Schwartz. All rights reserved
Sludge and grime that keeps the sear from “snapping back” into position, can also “slam-fires”. Therefore, it is also important to keep the sear track clean. In addition, you will want to avoid weakening the spring on the sear. To prevent this type of “slam-fire”, it is necessary to thoroughly clean the fire control group.
<<<<<<<<<
Multiple Discharge Problems Attributable to Poor Cleaning: In addition to “slam-fires” caused by a seized or dirty firing pin, many believe that the design of the SKS trigger group can cause unintended “repeats” (even complete magazine dumps) when the trigger mechanism is not cleaned properly.
In any semi-automatic firearm, one of the most essential parts of the firing, ejection, reloading cycle is the cocking of the hammer for the next shot. The SKS uses a “sliding” sear, which has to "snap back" in time to catch the hammer. However, if the trigger group is full of cosmoline, or dirt, powder residue, or other “gunk” the sear can be sluggish getting back to its original position in time to catch the hammer. If this happens, the hammer will not be caught by the sear and will follow the bolt forward as it chambers the next round. This can cause doubles, triples or even full magazine dumps—an unsafe situation indeed. While not technically the same thing as a “slam-fire”, it is a potentially dangerous situation and for this reason proper cleaning of the SKS trigger group is a must.
When you remove the trigger group, make sure that the area around the sear and hammer is completely free of all grime and gunk. Also, do not use heavy grease or excessive oil in the sear/hammer area. Too much lubricant can actually attract dirt and grime. A light application of CLP or other suitable free flowing light gun oil will provide suitable lubrication.
The info can be found here:
http://members.ispwest.com/LESchwartz/SKS-FAQ.html#CLEAN_FCG_DISCUSS
Be careful,
Jim