Under some circumstances, when posting an answer and using the browser's back button, hitting the 'Post Your Answer' button again results in the answer being posted twice. This happened to me here by accident; I thought I was editing my original answer. I was able to reproduce it here:
(note the orange banner which appears halfway, AFAICR it didn't appear when I first experienced the bug on Meta Stack Overflow)
This bug report looks similar: I was posting an answer, but it ended up appearing twice under a different question - bug? but it has status-norepro so I'm not sure.
I would expect the Answer form to have some kind of GUID which prevents a form from being submitted twice, or that hitting the back button would empty the form. Either way would be a good solution to prevent this behaviour.