Hi 2Kids1Bike,
"...but a "single" flower would likely produce another single? "
If you pollinated the single with its own pollen or the pollen from another single zinnia, then yes, the "single" flower would likely produce another single. If you pollinated the "single" with pollen from a "double" you
could get doubles from the single. The degree of "doubleness" can vary a lot. This specimen was
very double, perhaps too much so for its own good.
I noticed that the doubles in your photos are not extremely double. I am not as much concerned with singleness or doubleness as I am with the shape of the individual petals.
I am interested in different flower forms in zinnias, and there are some interesting forms available commercially, such as the Zinderellas. They could produce some interesting results when crossed with other zinnias.
ZM