One potential source of this problem is having two different .aspx files pointing to the same .aspx.vb file.
Another potential source is if you have copies in the same directory of the same .aspx and .aspx.vb files. For example you copied the files in windows explorer(copy of ... .aspx) to save your work as some point. Delete or move those copies out of the webapplication folders.
You could also try some of the steps in this post:
http://forums.asp.net/p/976740/1240521[
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