Never seen it on a Cosmos, but it probably does occur - its mostly likely down to something called Proliferation, where flowers try, or actually do, grow out of another flower, sometimes with a triple decker effect. Caused by some sort of injury (frost, physical damage) or a virus.

Occasionally, over-fertilization--especially with nitrates--can cause it, too.

If you look at my avatar, cosmos can do some wonderful things. Like my twin headed flower. Infact there are an awful lot of flowers that can fasciate or proliferate. as Bamboo says.

Thanks to all for replies from Glassman