What are the risk factors of double eyelid surgery?

Double eye surgery. I think you mean upper and lower eyelid surgery! this is done quite often by either competent plastic or an ophthalmologist. You must your eyes cheched for good tear formation and no dry eye problem. Competent surgeon will know how much skin to remove and avoid complcations.
Double eyelid crease. Double eyelid surgery is a popular procedure among asian cultures. A crease is created in the upper eyelid to help open the eyes. Most common side effects are temporary and last a few days. Some risks may include pain, bleeding, infection, scar, or asymmetry. Another risk the loss of the eyelid crease and requirement to repeat the procedure.
Asymmetry. The most common complication is probably asymmetry. In the immediate post op period bleeding or infection can occur. Scarring, lid malposition are also unlikely possibilities.
Dry eyes. Before you undergo the double eye lid surgery which seems like the upper and lower lid surgeyr. Make sure you have good quality tears, so that you do not end up with dry eye syptoms for your age the tear quality is likely to be good. Of course with any surgery the infection is a possibility. Good luck to you.