I filed same return twice by mistake, but 2nd one was rejected...why

[KatherineC]

You are only allowed to e-file one return using the same social security number.

However, you should check your rejection notice to make sure the rejection error was for filing a second time.  If by chance the first e-file was rejected, then the second e-file was rejected because the original e-file error(s) weren't corrected.