main do not call addAuthor() twice

Before:
"ksshaskpass --author
ksshaskpass was written by:
    Armin Berres <armin@space-based.de>
    Hans van Leeuwen <hanz@hanz.nl>
    Pali Rohár <pali.rohar@gmail.com>
    Armin Berres <armin@space-based.de>
    Hans van Leeuwen <hanz@hanz.nl>
    Pali Rohár <pali.rohar@gmail.com>
Please report bugs to armin@space-based.de."

After:
"ksshaskpass --author
ksshaskpass was written by:
    Armin Berres <armin@space-based.de>
    Hans van Leeuwen <hanz@hanz.nl>
    Pali Rohár <pali.rohar@gmail.com>
Please report bugs to armin@space-based.de."