How can I prove that my article has not been submitted to another journal? I just submitted a paper to some journal. Since they didn't specify a template, I used the IEEE template. Unfortunately, they immediately rejected my paper because they think I already submitted the same paper to one of the "IEEE Transactions" journals. I know for sure that's not the case. But how I can prove to them that my paper is not submitted to another journal. Is there a database for a list of submitted papers?
If I understand you correctly, they thought it was a double submission because the template you used included the header "IEEE Transactions on ..."?

Email the editor and explain that there has been a misunderstanding. If you make it clear that you only used the template because there was no template specified and you liked that one, a reasonable editor will probably at least remove you from their blacklist, unless they have other reasons to find your submission shady. They may or may not agree to consider your submission after all.

As a side note, "IEEE Transactions" is not even a journal. There are many "IEEE Transactions on &lt;Topic&gt;". If the template really only said "IEEE Transactions" you could note this in your explanation.