Can you tell me if my SSN number has been filed already?

If you or someone filed a return using your ssn as the primary one you might be able to check it here,
<a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/tax-tools/efile-status-lookup/">https://turbotax.intuit.com/tax-tools/efile-status-lookup/</a>

my parents filed me on their tax return, so I put someone can claim me and file for me, so why is it being rejected and saying my ssn has been filed already?

No one can tell you if your SSN has been used to file a return.  If you need to file a return your return will be rejected if your SSN has already been used on another return.  If someone else did file with your SSN & you file a return, the IRS (& State) would contact the both of you to find out who has the right to file the return.