diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b51cade..0efadcf 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -265,7 +265,12 @@
         if: contains(matrix.extra, 'vimtags')
         run: |
           # This will exit with an error code if the generated vim tags differs from source.
-          git diff --exit-code -- runtime/doc/tags
+          (
+            cd runtime/doc
+            git diff --exit-code -- tags
+            make html; rm *.html tags.ref;
+            test -f errors.log && exit 3;
+          )
 
       - name: Generate gcov files
         if: matrix.coverage
diff --git a/runtime/doc/maketags.awk b/runtime/doc/maketags.awk
index c6b2cd9..20b2b08 100644
--- a/runtime/doc/maketags.awk
+++ b/runtime/doc/maketags.awk
@@ -21,6 +21,13 @@
 	gsub(/%/,"\\&#37;");
 
 	nf=split($0,tag,"	");
+	if (counttag[tag[1]] > 0)
+	{
+		print "==============" > "errors.log"
+		print "Duplicate Tag " tag[1] > "errors.log"
+		print "==============" > "errors.log"
+	}
+	counttag[tag[1]]++
 	tagkey[t]=tag[1];tagref[t]=tag[2];tagnum[t]=NR;
 	print $1 "	" $2 "	line " NR >"tags.ref"
 	n=split($2,w,".");