Friend statement of singleton causing linker errors - singleton

Question

I get linker errors when building my program. The problem seems to be my NetworkManager singleton gets pulled into Networkable with the friend statement. I read that this can happen when having the instance declaration in the .hpp file, but if the instance() function isn't being inlined in some way this shouldn't be my case.
I found similar questions here, but no one that answers my problem. Any suggestions?
// Networkable.hpp
#pragma once
class Networkable
{
friend class NetworkManager;
public:
....
//NetworkManager.hpp
#pragma once
class Networkable;
class NetworkManager
{
public:
static NetworkManager &instance();
~NetworkManager();
void registerNetworkable(Networkable *networkable);
void unregisterNetworkable(Networkable *networkable);
void update();
...
// errors:
1>NetworkManager.obj : error LNK2005: "public: __thiscall NetworkManager::~NetworkManager(void)" (??1NetworkManager##QAE#XZ) already defined in Networkable.obj
1>NetworkManager.obj : error LNK2005: "public: static class NetworkManager & __cdecl NetworkManager::instance(void)" (?instance#NetworkManager##SAAAV1#XZ) already defined in Networkable.obj
1>NetworkManager.obj : error LNK2005: "public: void __thiscall NetworkManager::registerNetworkable(class Networkable *)" (?registerNetworkable#NetworkManager##QAEXPAVNetworkable###Z) already defined in Networkable.obj
1>NetworkManager.obj : error LNK2005: "public: void __thiscall NetworkManager::unregisterNetworkable(class Networkable *)" (?unregisterNetworkable#NetworkManager##QAEXPAVNetworkable###Z) already defined in Networkable.obj
1>NetworkManager.obj : error LNK2005: "public: void __thiscall NetworkManager::update(void)" (?update#NetworkManager##QAEXXZ) already defined in Networkable.obj

Answer 1

Right, I'm ashamed to say I found the answer. I had included NetworkManager.cpp instead of .hpp in Networkable.cpp.
Guess that's what happen when late-night coding...
But it does somewhat raise a valid and easy to make typo - writing .cpp instead of .hpp and not "seeing" the problem. It's much easier to spot a typo with just .h, and makes me question the .hpp file naming standard...