hmm... I think I wrote that example out for you.
Anyway, I just tested it, and there seems to be no problem.
Basically, you have at least 4 pages:
artists.php
artists_home.php
airgate.php
404.php
in their respective directories.
artists.php should contain the code given, and pretty much nothing else, assuming the other pages are complete.
What might have happened is that you have some code on the other pages (or perhaps artists.php) that is causing some page to be included more than once, or appear to be included more than once.