diff --git a/map_data_rules.mk b/map_data_rules.mk
index ab68f20360..3244d6e8db 100644
--- a/map_data_rules.mk
+++ b/map_data_rules.mk
@@ -36,6 +36,9 @@ $(LAYOUTS_OUTDIR)/layouts.inc $(LAYOUTS_OUTDIR)/layouts_table.inc $(INCLUDECONST
 
 # Generate files that depends on data that's distributed across the map.json files.
 # There's a lot of map.json files, so we print an abbreviated output with echo.
-$(INCLUDECONSTS_OUTDIR)/map_event_ids.h $(DATA_SRC_SUBDIR)/heal_locations.h: $(MAP_JSONS)
+# We're also not using a pattern rule, and we only want this to run once for both targets,
+# so we use a separate target 'event_constants'.
+$(INCLUDECONSTS_OUTDIR)/map_event_ids.h $(DATA_SRC_SUBDIR)/heal_locations.h: .event_constants ;
+.event_constants: $(MAP_JSONS)
 	@$(MAPJSON) event_constants firered $^ $(INCLUDECONSTS_OUTDIR)/map_event_ids.h $(DATA_SRC_SUBDIR)/heal_locations.h
 	@echo "$(MAPJSON) event_constants firered <MAP_JSONS> $(INCLUDECONSTS_OUTDIR)/map_event_ids.h $(DATA_SRC_SUBDIR)/heal_locations.h"