Do not add zerotier-one user if it does not exist (Debian)

debian/postinst

@@ -2,7 +2,9 @@
 
 case "$1" in
   configure)
-      adduser --system --group --home /var/lib/zerotier-one --no-create-home zerotier-one
+      if ! id zerotier-one >>/dev/null 2>&1; then
+        adduser --system --group --home /var/lib/zerotier-one --no-create-home zerotier-one
+      fi
       ;;
 esac