Check that admin sudoer doesn't already exist

src/freedombone-utils-setup

@@ -108,7 +108,9 @@ function initial_setup {
 }
 
 function admin_user_sudo {
-    echo "$MY_USERNAME  ALL=(ALL) ALL" >> $rootdir/etc/sudoers
+    if ! grep "$MY_USERNAME  ALL=(ALL) ALL" $rootdir/etc/sudoers; then
+        echo "$MY_USERNAME  ALL=(ALL) ALL" >> $rootdir/etc/sudoers
+    fi
 }
 
 function search_for_attached_usb_drive {