Only one way to find out...grow it out. Genetics, your odds are not good...but it may be a good mom. My experience with these is that the offspring will have some doubles. If you keep crossing these double offspring you should increase the genetic chances of double flowers. If you keep track of the offspring seedlings you may find one that is a superior mom and dad.

You could just buy a double, but...thats easy, anyone can do that.

Grow the double out, explore your plants, you have a single hellebore in your garden that has some double genes.

Double X Double can give a fair number of doubles but Double X Single can give anything. Many of the doubles in my garden seem self-sterile, but a few are very self-fertile. I have doubles from 3 or 4 original sources and all seem to have different growth habits and genetic backgrounds.

George