Is it acceptable to post a story that I've already posted on another site? I have one that fits the conditions here to a T.

Kinda backwards for me. Every so often, when I see a person out in public, I start imagining their back story, just making one up that seems to fit their look or the situation I saw them in. Something about them just triggers that. Or I see a picture online, and something about it - the setting or a look in their eyes or whatever - hits me, and a character and plot pop into mind. I mix and match in traits from people I know, or know about, along with just making things up.