It puts the null terminator at the end of both strings.

Dear Dansaki,

I ve noticed that you store result into static char variable complement;

I would try t store it into char * ptr_complement.

Otherwise you could breach some data in the memory space denoted for static variables

Glad you found it useful!

Psssst! Give this exercise a shot - you will enjoy it.

Happy coding!

I transferred your code to ideone.com and tested it for "abcdea" as the input.
It returns 0 as the value, while the correct solution must return 1 (because 'a' is a duplicate).

Please see https://ideone.com/C9s7JM

After you have fixed your logic, then bring it to codewars. In codewars, you don't need to define a main function.
Good luck,

Hi. Please use markdown tags to format your code or it is very discomfortable to read, see: how-to-format-code-in-markdown.

Moreover it's hard to help you if you don't give more elements of the problems you face: what fails? Do you have some error log, are the results different from what is expected?

This line: complement[strlen(dna)+1] += '\0'; is not a proper way to set a null string terminator.

Please read this: https://docs.codewars.com/training/troubleshooting#post-discourse