Be more explicit that it's only included once

eregon · eregon · commit 372017656b0b · 2023-01-11T17:12:06.000+01:00
* Avoids relying on the precise semantics of #extend.
diff --git a/lib/concurrent-ruby/concurrent/synchronization/object.rb b/lib/concurrent-ruby/concurrent/synchronization/object.rb
@@ -28,7 +28,7 @@ def initialize
       end
 
       def self.safe_initialization!
-        extend SafeInitialization
+        extend SafeInitialization unless safe_initialization?
       end
 
       def self.safe_initialization?
