On average, the "ideas" stemming from a population with not-insignificant overlap of habits and personal preferences will be drawing upon the same sources of inspiration: News items, TV shows or movies, other trending sources that multiple samples of the population could be following, etc. While most of the time this will only manifest as a "I like what you like" compatibility, sometimes it can result in decidedly specific ideas manifesting in otherwise unconnected individuals at around the same time.
'Tisn't unheard of. History has examples of inventions developing around the same time, unknown to each separate inventor, or stories written with broad similarities, again without each author's knowledge of the other… heck, there have been examples of two different people being born on same day, same name, eerily similar backgrounds, etc. With seven billion people on the planet, the odds that multiples will have similar thoughts or ideas bouncing around in their heads at roughly the same time isn't too shabby.