Worry not, Sam. The publisher of the magazine or anthology does not ‘own’ anything other than the right to publish it once. After that you are free to offer it out again or to change it in whatever way you wish. I would suggest that, if the competition rules imply anything more than this, you think long and hard before submitting to them.
Publishing restrictions of this nature only really come in to play with full-length novels. When you sign a contract with a publisher you give them exclusive rights to publish that novel – and a defined number of subsequent ones, depending on the contract – until the contract expires, at which point you, hopefully, agree a new deal with them.
Good luck and keep dreaming. That’s how every successful writer starts out.
ee.
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