I’ve been getting this error every now and then.
Usually this problem is caused by having 2 controls with the same name. It can be that they are in different folders as well.
I came across this post today:
Well, a quick search online told me that this might be a result of a known bug in the compiler. The fix was easy, you have to compile your app in non-batch -mode. How do you do this, I hear you asking. Simple: enter the compilation section of your web.config, and set batch=”false”, as so:
<compilation debug=”true” batch=”false”>