Magazines dislike simultaneous submissions, as they may then waste time agreeing to publish a poem when another magazine may already have picked it up. It can also take up to 6 months for a submission to be responded to, unless they really hate it and it’s an obvious “no”. The general advice given is therefore to keep a careful track of what poems are where, to avoid running aground. Magazines typically publish around 1% of submissions (so many poets, so little room).
So, in a fit of anality, I have a spreadsheet. There is a lot of unhappy red, and now a small splash of jolly green.