For the longest time I thought it was a bug in the 620, since that was the first Garmin I had with wifi upload. Many users complained of this behavior (since at least January 2013 when I first set up wifi on my 620), and felt it was a culprit in erratic wifi uploading, but it was never solved.
Since I got the 920XT a couple of weeks ago, it's doing the same thing. So my latest guess is that it's a bug in Express that somehow duplicates and re-installs the networks.
I have three wifi networks that I'd like my Garmins to recognize. When I opened Express before starting to write this post, it showed my 920 had two iterations of one of them, and three iterations of another (the one I added most recently hadn't been duplicated ... until the next time). So I deleted the duplicates, synced the watch, disconnected it from my computer, quit Express, then re-connected the 920 and opened Express ... and now it's showing two copies of each of all THREE networks.
Will this be fixed?