I think I have come across a bug in how Doxygen handles forward 
declarations of class templates. Suppose that I have the following file:

namespace N
{
template <class T> class C;

/** \brief C */
template <class T> class C
{
public:
/** \brief method */
     void method() {}
};
} // end namespace N

When I process it with Doxygen 1.8.2, the generated class list 
("annotated.html") contains not only the N::C class template, but also a 
spurious entry "C" outside any namespace. The latter disappears if I 
remove the forward declaration of C.

The same problem occurs if a forward declaration of a class template is 
found in a different source file than its definition, and each such 
declaration produces a new spurious entry in the class list.