A post can only be reported once. I think OP meant that they were able to report the post, meaning no one prior to them reported the said post.
A post can be reported multiple times as long as it is not yet deleted. If a mod takes action, all reports linked to it would get marked as good.
I think that a post can be reported once by one account, since my report for this same post also tells it was a good report.
A post
should be reported only once, but it can be reported more than once by one account. If you send out duplicate reports one would likely be marked as bad. But if you make 2 unique reports on one post, such as spamming and plagiarism, they would both be handled differently.