In my experience, you can have single and double flowers on the same plant. Sometimes at the same time, or sometimes it may bloom as a double in one bloom cycle and as singles in another cycle. That has been the case with my East Wind, one of my favorite plants. It usually blooms with double flowers, but the flowers were singles in one cycle. I just checked it, and right now it has mostly doubles, but I did see a single. The blooms on my Mac's Exquisite Extravaganza are usually single, but I see a double bloom on it right now, along with the singles. I think, without any great certainty, that the single pansy type of bloom usually blooms true as a single. Does your plant have a name, or is it a NOID?

Thank you for your reply! It's a NOID, unfortunately. It came from a local grocery store.

Ah, I've gotten a few wonderful AV's at grocery and big box stores. But be sure to isolate them for several months to make sure you haven't brought in any unwanted critters.