Since the last changes in flagging I can not flag a post more then once. Previously if the flag I raised became invalid(e.g. if the author edits the post) I flagged the post again to say "my previous flag is not valid", both flags being marked as disputed. I believe this saved the moderators some work.
Now that I can not flag a post more than once it is not possible to remove an invalid flag. I did not like the old system too much, but I don't think the new way is much better. For instance if I accidentally put a wrong flag on a question I can not remove it. I think the best option is to enable a user to flag a post more than once but each flag should mark the previous ones as
disputed. This will save the moderators some work of looking into invalid flags.