The quick outline navigator shows namespaces twice.

If you have a header:

namespace a
{
namespace b
{
namespace c
{
struct s
{
  void f();
};
}
}
}

and in the source file:

namespace a
{
namespace b
{
namespace c
{
void s::f()
{
   // click here or place cursor here
}
}
}
}

Then the quick outline navigator will show: a::b::c::a::b::c::s::f()

On Debian Buster, Xfce, with kdevelop 5.3.1 AppImage.
This issue is redundant since many versions of kdevelop.