How to fix annoying Duplicate class error message in NetBeans 7.+

I don't know what I did, but suddenly NetBeans started to complain that some of my classes are duplicate?
To fix this issue, close NetBeans, delete NetBeans cache, start NetBeans again.

Location of the NetBeans cache is in

C:\Users\<UserName>\AppData\Local\NetBeans\Cache

References:
NetBeans 7.0 Duplicate class error message
How to clear the Cache in NetBeans