diff --git a/extras/update_vpn_users.sh b/extras/update_vpn_users.sh
index 49624eb..0df05da 100644
--- a/extras/update_vpn_users.sh
+++ b/extras/update_vpn_users.sh
@@ -82,6 +82,10 @@ case "$VPN_USERS $VPN_PASSWORDS" in
     ;;
 esac
 
+if printf '%s' "$VPN_USERS" | tr ' ' '\n' | sort | uniq -c | grep -qv '^ *1 '; then
+  exiterr "VPN usernames must not contain duplicates."
+fi
+
 clear
 
 cat <<'EOF'