"Noticed" might not be the right word. Let's say I'm reviewing a question that could have issues (off-topic, opinion based etc) but someone has already commented or voiced a close-vote. (I can't see close-vote queue)
Do you pick "No action needed" knowing the question does need work because others have noticed it, or do you flag it anyway even though others might have already done so ?
Being SE a community-driven website I would flag it regardless if it has already been noticed. The more voices back up a close/reopen/delete opinion the more that opinion is likely to be correct. – JoErNanO
This. There's no downside not flagging something that needs to get rectified, flagged already or not. One and the same flag on the same post from different people appear grouped together in the flag queue, so we as moderators are not overwhelmed -- if that is at all your concern. In fact, having flags from more people actually helps sometimes -- personally, I tend to err on the side of caution when reviewing a borderline post, but presence (or lack thereof) of multiple flags from frequent reviewers at times has weighed on my decision on what to do with a particular post.
If anything, I feel there are sometimes too few flags raised by the Travel-SE community -- particularly on popular network-wide questions that often need serious cleanup and occasional protection. We as moderators don't have time to look through all posts on a particular day -- not anymore, thanks to the steady growth of the site over the year -- and we rely on our fellow community members to keep the place clean and tidy.
common courtesy. The op should know why their post is being flagged so that they can act upon it.