Duplicate Artist entries when mixing MP3s and WMAs

My music is in a mix of MP3 and unlocked WMA format.  I’ve noticed that if I load songs in both formats for the same artist, the artist’s name is duplicated under the ‘Artist’ selection when searching for music on the Clip.  The ID tags are exactly the same, and the songs show grouped under the same Artist in any other player or software package I use.

anyone else seeing this?