Previously published stories

You need to check the guidelines of the market you are sending it to and the market it was reprinted in. Some markets won't reprint unless it's a year old, for instance. There's usually clear indication of this in the 'Rights' section of the guidelines.

I may be wrong, but I would think that the market it was first reprinted in retains the rights for a specific amount of time, also? For instance, one of the markets I have a story published in holds anthology rights unless I specifically request it not be included in their anthology. I would assume that if it's included, I might have trouble getting a reprint right on it. You might want to check on that.

All depends on the contractual obligations, in regards to whether or not you can submit it to additional reprint outlets...

If you can get it reprinted again, I'd reference the more popular of the two magazines. If it was the first magazine the story was in, then mention it "first appeared in..." If it was a later one, then it "previously appeared in..."

But that could just be me...

From what I've done in the past, I always reference where it first appeared, when, and page number. Then it's second serial reprint rights. Also, check with the guidelines where you're submitting it to now.

I believe you have to let them know if it has been published before.