I accidentally created a flag as duplicate for this question:
I was in the input to select a duplicate. Typed some words and pressed enter. It automatically selected the first entry. It didn't ask me anything and created the flag.
That said, I believe it could be good to be able to cancel our own flags. I see it as active in when I click the flags I created but there is no way to remove it. As I see it, it should be possible to remove our own flags as long as they are active. When the flag get rejected or accepted, it should not be possible to cancel it.
One other way to handle flags could be to show a flash popup just after a flag has been created with a link to cancel it. If you let the flash message go away, then it won't be possible to cancel it or you'll have to go in your profile to remove (it if such feature could be added).