Example
header.h:
---
#ifndef HEADER_H
#define HEADER_H

using namespace std; 

#endif /* HEADER_H */
---
                                      
header.h is included by: source_1.cpp, source_2.cpp, source_3.cpp, source_4.cpp

In editor we get 4 similar hints (google-global-namespace-in-headers), the only difference is CU.

It's better to show 1 hint, but with message like: "Occured in source_1.cpp, source_2.cpp, ..."