diff --git a/main/src/main/scala/sbt/Project.scala b/main/src/main/scala/sbt/Project.scala
index 789cb709f..5f867dd94 100755
--- a/main/src/main/scala/sbt/Project.scala
+++ b/main/src/main/scala/sbt/Project.scala
@@ -152,7 +152,7 @@ private[sbt] object CompositeProject {
         case None                    => p
       }
     }
-  }
+  }.distinct
 
 }