ensure uniqueness of the IDs

notes/cvtdat.sh

@@ -196,6 +196,15 @@ function anchorIt {
 	    "item identifyer ${id@Q} is not a valid ASCII xml:id NCName"
 	[[ $id = [A-Za-z]*([A-Za-z0-9_:.-]) ]] || die \
 	    "item identifyer ${id@Q} is not a valid HTML 4 id attribute"
+	local -l idlc=$id
+	local idid=ID_${idlc//-/D}
+	idid=${idid//./P}
+	[[ $idid = ID_+([0-9A-Z_a-z]) ]] || die \
+	    "internal consistency error on ${idid@Q}"
+	nameref idv=$idid
+	[[ -z $idv ]] || die "ID ${id@Q} already used as ${idv@Q}"
+	idv=$id
+
 	idhtm=${|tohtm "$id";}
 	idltx=${|totex "$id";}
 	if [[ -z $lbl ]]; then