What Is the 'Duplicated URL Entry' Error?
It means the link you entered has already been submitted to this Submit URL Action, either by you or by another entrant. Each link earns entries only once, so submit a different one, like the link to your own post.
Gleam simplifies a link before it checks it. It ignores http or https, and it strips the #hash and any URL parameters the host hasn't allowed. So https://yourdomain.com/post?ref=1 and http://yourdomain.com/post count as the same link.
If you're the host and want entrants to be able to submit the same link, open the Action on your How to Enter tab and switch on Accept Duplicate URLs. To keep a parameter, add it to Allowed URL Parameters (comma separated and case sensitive). Both settings are covered in Submit URL.