Is File Upload class creates unique file name when encrypt_name set true?

encrypt_name does not guarantee a unique file name. That said, the chance of a name collision is incredibly small. But a small chance is not the same as no chance.

You might be interested to know that do_upload() calls set_filename() and set_filename() will append a number to the end of the filename to avoid overwriting a pre-existing file. This happens for any file upload encrypted name or not.