Converting "0xUsernames"
05AB1E, 4 bytes

D1ìH


Explanation

D    Duplicate input
 1ì  Prepend 1
   H Interpret as hexadecimal and implicitly display the value in base 10


If the input has invalid hex characters, H won't push anything so the last value on the stack will be the duplicated input, that's why the program prints its input in case of invalid input.

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