In the following image you see a question posted as an answer. Normally, I'd just flag this question as not an answer and move on. However, I ran into this question in the Low Quality Posts queue and I already recommended its deletion. Should I also go and flag it as not an answer?
You can, but it's not a good use of your time. The "not an answer" flag puts a post in the Low Quality Review Queue. If the post is already under review there, the flag isn't of any use (it doesn't do any harm, either).
I don't know if there is general SE guidance on this, but I would say yes — clear cut cases seem like a good place for flags. This site is not at the point where moderators need to heavily triage flags, as yet. That said, I also go fully through most queues regularly, so a flag is not strictly necessary.