diff --git a/dom/plugins/PluginInstanceChild.cpp b/dom/plugins/PluginInstanceChild.cpp
index 5db10429ec3..48dbf5a84a6 100644
--- a/dom/plugins/PluginInstanceChild.cpp
+++ b/dom/plugins/PluginInstanceChild.cpp
@@ -1393,9 +1393,13 @@ PluginInstanceChild::SetWindowLongAHook(HWND hWnd,
         GetProp(hWnd, kPluginInstanceChildProperty));
 
     // Hook our subclass back up, just like we do on setwindow.   
-    self->mPluginWndProc =
-        reinterpret_cast<WNDPROC>(sUser32SetWindowLongAHookStub(hWnd, nIndex,
-            reinterpret_cast<LONG_PTR>(PluginWindowProc)));
+    WNDPROC currentProc =
+        reinterpret_cast<WNDPROC>(GetWindowLongPtr(hWnd, GWLP_WNDPROC));
+    if (currentProc != PluginWindowProc) {
+        self->mPluginWndProc =
+            reinterpret_cast<WNDPROC>(sUser32SetWindowLongAHookStub(hWnd, nIndex,
+                reinterpret_cast<LONG_PTR>(PluginWindowProc)));
+    }
     return proc;
 }
 
@@ -1422,9 +1426,13 @@ PluginInstanceChild::SetWindowLongWHook(HWND hWnd,
         GetProp(hWnd, kPluginInstanceChildProperty));
 
     // Hook our subclass back up, just like we do on setwindow.   
-    self->mPluginWndProc =
-        reinterpret_cast<WNDPROC>(sUser32SetWindowLongWHookStub(hWnd, nIndex,
-            reinterpret_cast<LONG_PTR>(PluginWindowProc)));
+    WNDPROC currentProc =
+        reinterpret_cast<WNDPROC>(GetWindowLongPtr(hWnd, GWLP_WNDPROC));
+    if (currentProc != PluginWindowProc) {
+        self->mPluginWndProc =
+            reinterpret_cast<WNDPROC>(sUser32SetWindowLongAHookStub(hWnd, nIndex,
+                reinterpret_cast<LONG_PTR>(PluginWindowProc)));
+    }
     return proc;
 }