diff -r ac13bf1967b5 Lib/threading.py
--- a/Lib/threading.py	Thu Sep 15 14:15:29 2016 -0400
+++ b/Lib/threading.py	Thu Sep 15 21:32:21 2016 +0200
@@ -838,9 +838,11 @@
         if not self._initialized:
             raise RuntimeError("thread.__init__() not called")
 
-        if self._started.is_set():
-            raise RuntimeError("threads can only be started once")
         with _active_limbo_lock:
+            if self._started.is_set():
+                raise RuntimeError("threads can only be started once")
+            if self in _limbo:
+                raise RuntimeError("thread currently starting")
             _limbo[self] = self
         try:
             _start_new_thread(self._bootstrap, ())