From Cory Riddell, "I added a line to clear the threads vector prior to populating it in

Viewer::getAllThreads(). This is consistent with what happens in
Viewer::getOperationThreads()."

src/osgViewer/Viewer.cpp

@@ -1004,6 +1004,8 @@ void Viewer::getViews(Views& views, bool onlyValid)
 
 void Viewer::getAllThreads(Threads& threads, bool onlyActive)
 {
+    threads.clear();
+
     OperationThreads operationThreads;
     getOperationThreads(operationThreads);