[Dev-C++] Problem with header, skipping lines?, or what???

I'm just making a project, there I have two sources and one header, 
which is includedin both source files.

The header is
#ifndef CGMSTRING_H
#define CGMSTRING_H

#include <string>

using std::string;

class String : public string{
etc
#endif

Then the two source files including cgmstring.h. One have the main 
function and the other is the implementation of a class, which has its 
header.
The problem is that either compiling trhough a makefile or thropugh 
building a project with Dev, the header is included for both sources in 
their object files, so when it's the time for the linker it says all the 
functions of cgmstring.h (three constructors, one for c_str() and 
another vc_str()) are declared twice: in main.o and in class.o. But if I 
say, when compiling to .o in either one of the sources, -D CGMSTRING_H, 
then the compiler reports that String has not been declared!.
It's the first time I find this, always before it worked fine, why 
happens this now, what can be the problem?