diff --git a/web/Makefile b/web/Makefile
index e53d091..aba48d1 100644
--- a/web/Makefile
+++ b/web/Makefile
@@ -1,18 +1,27 @@
 
 all: swdb.lst.sig
 
 swdb.lst: swdb.mac
 	awk '                                \
 	     ! /^#\+macro:/ {next}           \
 	     $$2 ~ /ftp_.*/ {next}           \
 	                    {print $$2, $$3} \
 	    ' swdb.mac >swdb.lst
 
-swdb.lst.sig: swdb.lst
+swdb.lst.sig: swdb.lst signcheck
 	gpg -sbu 0x249B39D24F25E3B6 swdb.lst
 
+signcheck: swdb.lst
+	@set -e; \
+        tmp="$$(awk '$$1~/^.*_ver/{print $$1}' swdb.lst \
+            |sort|uniq -c|sort -nr|head -1|cut -b 1-7)"; \
+	if [ "$$tmp" -ne 1 ]; then \
+	  echo "ERROR: Duplicate version numbers found" >&2; \
+          exit 1; \
+        fi
+
 upload: swdb.lst.sig
 	scp swdb.lst.sig swdb.lst playfair.gnupg.org:/var/www/git/versions.gnupg.org/htdocs/
 	scp swdb.lst.sig swdb.lst webbuilder@trithemius.gnupg.org:/var/www/www/www.gnupg.org/htdocs/
 
-.PHONY: upload all
+.PHONY: upload all signcheck