How to Use PostThreadMessage() (Unexpected Results) - c++

Question

First, I'm new to multithreading. So, if there is a better way than my approach I would like to hear.
I'm injecting my code into another process. I created a CBT hook to get notified when new windows are created. When my target window created, I get notification by CBT hook and create a thread, and at the end this thread calls PostThreadMessage() function to send data/notification to another thread of me (which is inside same/target application, too).
But this doesn't work as expected. My receiving thread gets that message 4 times before I even send it first. Then after I send it, it doesn't get it this time.
Let's come to code.
This is how I create receiver thread.
#define MM_MY_MESSAGE (WM_APP)
// Also tried with different macros
// #define MM_MY_MESSAGE (WM_APP + 999)
// #define MM_MY_MESSAGE (WM_USER)
// #define MM_MY_MESSAGE (WM_USER + 999)
gReceiverThreadHandle = CreateThread(NULL, 0, ThreadReceiver, NULL, 0, &gReceiverThreadId);
// global variables
This is my receiver thread, which also uses WM_COPYDATA messages.
DWORD WINAPI ThreadReceiver(LPVOID lpParam) {
MSG msg;
HWND hwndReceiver = CreateReceiverWindow();
// If necessary I can post this function later
// This window is created for WM_COPYDATA messages
// Not important for PostThreadMessage()
if (!hwndReceiver) {
std::cout << "Receiver window can not be created!" << std::endl;
return 0;
}
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}
This is WndProc for WM_COPYDATA and MM_MY_MESSAGE.
LRESULT CALLBACK ReceiverWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam) {
switch(message) {
case WM_COPYDATA:
// do something
break;
case MM_MY_MESSAGE:
std::cout << "PostThreadMessage received" << std::endl;
return 0;
}
return DefWindowProc(hwnd, message, wParam, lParam);
}
This is how I call PostThreadMessage():
DWORD WINAPI AnotherThread(LPVOID lpParam) {
// This thread gets created inside CBT hook which is unrelated
// etc.
PostThreadMessage(gReceiverThreadId, MM_MY_MESSAGE, NULL, NULL);
std::cout << "PostThreadMessage send" << std::endl;
// etc
return 0;
}
After I run my code, output shows,
PostThreadMessage received
PostThreadMessage received
PostThreadMessage received
PostThreadMessage received
PostThreadMessage send
This is clearly not what I wanted. What is wrong here?