Visual C++ :: Multiply Defined Symbols When Linking To DLL

I'm building two DLLs - let's call them DLL_A and DLL_B. DLL_A builds as a standalone entity but DLL_B needs to link to the lib file for DLL_A (i.e. it imports some functionality from DLL_A). While linking DLL_B I see lots of errors taking the following form (bear in mind that port.cpp and port.h are source files in DLL_B:-

DLL_A.lib(DLL_A.dll) : error LNK2005: "public: bool __this call
std::vector<class std::basic_string<char,struct std::char_traits<char>,
class std::allocator<char> >,class std::allocator<class std::basic_string<char,struct std::char_traits<char>,
class std::allocator<char> > > >::empty(void)const " (?empty@?$vector@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@V?$allocator@V
?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@@std@@QBE_NXZ)

already defined in port.obj

I'm not sure if I'm reading that correctly but to me, it seems to be saying that some STL components are somehow getting exported from DLL_A (std::vector maybe?? Or std::string??) and that they conflict with similar objects already in port.obj. Sure enough, when I used dumpbin /EXPORTS on DLL_A there did seem to be some evidence that that was true. So my next step was to examine the source code for port.obj. Of course, strictly speaking I should be examining some code from DLL_A but it has hundreds of source modules so I figured that I should start by identifying whatever it is in DLL_B that's throwing up the conflict (since I at least know which module the conflict is in!).

When I examined the source files for port.obj, std::string seems to get used quite often - but fortunately I could only find one occurrence of std::vector.

In port.h it occurs here:-

Code:
class DLL_B_API Port : public boost::noncopyable {
public:
// c'tors + d'tors
int get_connections (std::vector<std::string> &) const;
// rest of class

In port.cpp it occurs here:-

Code:
int Port::get_connections (std::vector<std::string> & c) const {
if (!port_engine.available()) {
c.insert (c.end(), _connections.begin(), _connections.end());
return c.size();
}
return port_engine.get_connections (_port_handle, c);
}

Is there anything in there that would be causing the above linker error? It's entirely possible that I'm looking in the wrong place but I suppose I've got to start somewhere....

I just found a possible clue in one of the header files for DLL_A, where I found this class declaration:-

Code:
namespace PBD {
class DLL_A_API Searchpath : public std::vector<std::string> {
// Whatever...
};
}

Might that be causing std::vector<std::string> to get exported?