# gcc --version
gcc (Ubuntu 4.4.3-4ubuntu5) 4.4.3

3 source files: ns1.h ns2.h test.cpp

---------------------------------------------------------
ns1.h:
#pragma once
class A
{
};
---------------------------------------------------------
ns2.h:
#pragma once
class A
{
};
---------------------------------------------------------
test.cpp:
namespace NS1
{
#include "ns1.h"
}
namespace NS2
{
#include "ns2.h"
}
int main()
{
    NS2::A a;
    return 0;
}
---------------------------------------------------------

# touch *   // important step, let ns1.h and ns2.h have same modification time
# gcc test.cpp
test.cpp: In function 'int main()':
test.cpp:11: error: 'A' is not a member of 'NS2'
test.cpp:11: error: expected ';' before 'a'

# gcc -c -E test.cpp
# 1 "test.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.cpp"
namespace NS1
{
# 1 "ns1.h" 1

class A
{
};
# 4 "test.cpp" 2
}
namespace NS2
{

}
int main()
{
    NS2::A a;
    return 0;
}

----------------------------------------------------
The NS2::A declaration isn't included in namespace NS2