One could say everyone plagiarized from each other. Or one could say humans tend to want to tell the same kinds of stories, kinda like how dystopia is popular because something about our brains loves end of the world stories.
Or one could say a similar event occurred in various parts of the world, a similar spirit.
Of course many of these similarities may just be made up, but that doesn't take away from the possible factual life those people lived or their teachings.
People like to add all kinds of silliness to everything. A story told of Buddha is that he walked when born. Probably not true, added by people, but this doesn't mean he was not real or take away from his message.
There is the real person. The real message, the spirit behind the person, and then there is all the fluff people add to it.
So, does it matter if Christ was not born of a virgin? I don't think so. Maybe it was added fluff, maybe not. I tend to think so, but his message, his spirit is what matters here, at least to me.