If the local track to be synced is on the server Spotify will naturally download this version. If you wish to change this to force your local file you will need to modify the track name (and id3 tag) to make it a little different.
Please use the search feature before posting a problem/bug/idea. What's a Spotify Rock Star, and how do I become one? Using Spotify Since March 10th 2009 Twitter: @mechanimal82