diff --git a/core/ninja.mk b/core/ninja.mk
index 2878514..5876ad8 100644
--- a/core/ninja.mk
+++ b/core/ninja.mk
@@ -116,6 +116,10 @@
 NINJA_ARGS += "-v"
 endif
 
+# Make multiple rules to generate the same target an error instead of
+# proceeding with undefined behavior.
+NINJA_ARGS += -w dupbuild=err
+
 ifdef USE_GOMA
 KATI_MAKEPARALLEL := $(MAKEPARALLEL)
 # Ninja runs remote jobs (i.e., commands which contain gomacc) with