not fail make if no duplicate

Makefile

@@ -186,6 +186,6 @@ up: $(PDFNAMES)
 .PHONY: check
 check:
 	@echo "Checking duplicates.."
-	@grep -Eon -r --include "*.tex" '(\b[[:alpha:]]{2,}\b) \1\b' .
+	@grep -Eon -r --include "*.tex" '(\b[[:alpha:]]{2,}\b) \1\b' . | true
 	@echo "Checking spelling.."
 	cat $(SRCNAMES) | aspell --conf-dir=$(THISDIR) --home-dir=$(THISDIR) list -t --dont-tex-check-comments -d en_GB-ize | sort -uf | sed -E '/.../!d'