It depends on 2 conditions. If the another contest has been closed and the concept is not the winner/runner up, yes, the designer may upload his/her previous concept to your contest. If the another contest is still an ‘Open Contest’, then the designer is violating our rule which uploading the same contest to two or more open contest simultaneously is prohibited.
If you find a designer do that thing, please report to us by reporting his/her design. You can click icon ‘!’ triangle below his/her design and choose ‘Report this design’, then follow the given instructions on your page.