What I do to avoid artist repeats is to have only one song by an artist in an active playlist at any given time. I divide my artists into general rotation randomized playlists by the number of songs they have in rotation and then rotate amongst those daily.

Songs repeating after six hours can be remedied by using 'active from active to' settings to switch to another playlist before the default six hour window, although this feature seems borked since the v3.2.10 update.

'active from active to' problems:
http://forums.centova.com/index.php?topic=4482.0
http://forums.centova.com/index.php?topic=4472.0