Since some flags are handled by non-moderators, they may be marked as "disputed" and effectively thrown away. I review a lot in Stack Overflow's Triage queue, so this happens to me with appalling frequency. (Disputed flags can appear outside of the Triage queue, of course, so I'm posting this in the network-wide meta.) I believe a lot of those flags are valid, so I go to re-flag, but then I get this:
I know I can raise a custom flag, but that would get rather tedious, and I'd be afraid of forcing mods to read yet more text, and of getting the flags declined with a message about using the normal reasons.
I propose that disputed flags not block the re-flagging of the post. There should probably be a guarantee that the second flag be handled by a real mod. A note in the flag dialog about an existing disputed flag might be appropriate.
"Once a flag is cleared from the mod queue, the same post cannot be re-flagged" and "Allow users to flag questions more than once" are status-completed, saying that flags can be re-raised after being looked at by a moderator, and disputed flags never get looked at by mods, so this seems to be a separate request and not a bug.