diff --git a/cmd/pom2mk/pom2mk.go b/cmd/pom2mk/pom2mk.go
index 5842f70..b4659f7 100644
--- a/cmd/pom2mk/pom2mk.go
+++ b/cmd/pom2mk/pom2mk.go
@@ -360,6 +360,7 @@
 
 	poms := []*Pom{}
 	modules := make(map[string]*Pom)
+	duplicate := false
 	for _, filename := range filenames {
 		pom, err := parse(filename)
 		if err != nil {
@@ -373,11 +374,15 @@
 
 			if old, ok := modules[key]; ok {
 				fmt.Fprintln(os.Stderr, "Module", key, "defined twice:", old.PomFile, pom.PomFile)
+				duplicate = true
 			}
 
 			modules[key] = pom
 		}
 	}
+	if duplicate {
+		os.Exit(1)
+	}
 
 	for _, pom := range poms {
 		pom.FixDeps(modules)