The type exists in both ”temp1.dll“ and ”temp2.dll"

I am getting the following error in my ASP.NET app: The type 'foo' exists in both ”temp1.dll“ and ”temp2.dll"
I have gone through threads like http://stackoverflow.com/questions/371426/asp-net-error-the-type-foo-exists-in-both-temp1-dll-and-temp2-dll and others but none of them have helped. 
Any idea what might be causing the error?