I did something a bit silly a while ago. I submitted the same story to two different places at the same time. The first was an SF magazine, the second was the Writers of the Future competition. Exact same story, 2 different places. At once.
I won't go into my reasoning for doing this, other than to say it was a spur of the moment thing, and I knew straight away it could potentially lead to problems. But I stuck with it anyway, figuring I'd just wait and see.
And the result surprised me.
The magazine snatched the story up for publication. This particular magazine is very hard to get into, and I was astonished when I found out they'd picked my piece.
WoF rejected it. Not even an Honourable Mention. Just a flat reject.
I find something extremely encouraging in this unintentional experiment. If I'd only submitted the story to WoF, I would quite likely have assumed I'd written a dud, and even though I would have sent it off to other places, it would still have been a huge discouragement. But it wasn't only sent to WoF; it was sent somewhere else and accepted.
To me, it's a huge reminder that a rejection doesn't necessarily mean there's anything wrong with your story. More often than not it simply means you submitted it to the wrong place. Pick yourself up, dust yourself off, and send it out again. And again. And again.
That's why magazines urge you to read at least one of their editions before you submit, so you can gauge what sort of stuff they print, and whether your piece "fits". I don't always do this, and I know I should. Maybe if I did I'd have a few less homeless stories floating about the place.
Anyway, undeterred by my competition result, I submitted another story for the new quarter of WoF. This time, it's a brand new piece, never submitted anywhere else. Is it good? Well, I kind of like it. Will it do well? I have absolutely no idea, because I still haven't read any of the WoF anthologies or seen any of the winning stories. I'm just working on the basis that the only sure way to fail is to never enter at all.
I'll let you know how I went in about 3 month's time!