Single combs aren't all that rare in all stocks, including show stocks. Often, it's either not noticed due to being mostly obscured by the crest... or nobody wants to admit it, lest people think their stock ain't so pure. Elephant in the room, basically.
There is another problem here, that isn't discussed much also. Huge majority of silkies actually have a modified rosecomb that looks like a walnut comb. (this in itself is not a problem, as they do look very convincingly like a true walnut on many good roosters)
The reason I bring that up is because roosters pure for rosecombs have much lower fertility than all roosters of other comb types- including rosecombed roosters NOT pure for rose comb. A rosecombed rooster with one copy of single comb is going to be far more fertile than a rooster pure for rosecomb.
So the overall effect is that those not-pure roosters are going to seemingly prove themselves as great studs and producing lots of fertile eggs. This has the effect of spreading that single comb floating around mostly hidden. I agree though with above advice, it would be preferable not to use any single combed birds for breeding. Increases the chance of it showing up again quite a bit if those birds are used.
It is strange, that in comparsion other rose combed breeders are much more open about it- single comb showing up in a Wyandotte is generally regarded as no surprise. Not desired, but understood as result of the pure rosecomb rooster fertility problem.