I flagged a post today, typed out a fairly thorough explanation, clicked the button... and got told that I'd already flagged this post (for something else, which I'd forgotten about) so this flag wasn't allowed. (This happens if the first flag is still active.) The dialogue with my explanation was gone at this point, so if I wanted to communicate with the moderators about that, I was going to have to start over. (In case you're wondering how this could happen, one flag was related to the post itself and one was related to the user -- there being no way to flag a user directly, you have to hang it off of a post. But I've also seen this with cases where the first flag hangs around unhandled for a week or two and I've forgotten about it.)
Since the system is going to detect that I've already flagged a post and reject a second flag, can it do that after I click the "flag" link instead of waiting until the end of the process? (Or gray out the link so I can't click it in the first place?)
2015-12-18: someone suggested to me in chat today that this had been implemented, so I tested again. The behavior has not changed: I flagged a post using "other", then flagged it again (also "other") before the first flag was handled, and the flag was rejected only after I'd typed the message. (I did this on a site where I'm a moderator so I could then clean up after myself.)