From Birnam Designs Wiki

Sometimes when a site update is uploaded, ASP.NET will recompile the modified classes in new modules/dll files. This causes a duplication that will create an error for the entire site.

To fix this, rename the bin directory to a temporary name. Then revisit the site -- you will see an error reflecting the misplaced directory. Go back and rename the directory to bin and revisit the site. You have tricked the server into recompiling the entire website, and the duplication error is fixed.