diff --git a/build.gradle b/build.gradle
index 887cd9ff5e09a680837b4299513ccb66ce7c8358..c1e53937abf49158c0b37ed8a38785ba7d371247 100644
--- a/build.gradle
+++ b/build.gradle
@@ -81,7 +81,7 @@ jar {
     from {
         configurations.runtimeClasspath.collect { return (it.isDirectory() ? it : zipTree(it)) }
     }
-    duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+    duplicatesStrategy = DuplicatesStrategy.FAIL
 }
 
 def relastFiles = ['./src/main/jastadd/common/MotionGrammar.relast', './src/main/jastadd/World.relast', './src/main/jastadd/LoadUnload.relast']