diff --git a/lib/thread.rb b/lib/thread.rb
index 09f2d58..6b47a9f 100644
--- a/lib/thread.rb
+++ b/lib/thread.rb
@@ -299,7 +299,7 @@ class SizedQueue < Queue
     @mutex.synchronize{
       while true
         break if @que.length < @max
-        @queue_wait.push Thread.current
+        @queue_wait.push Thread.current unless @queue_wait.include?(Thread.current)
         @mutex.sleep
       end