If you can identify an earlier question that is basically the same, vote to close the new question as a duplicate, giving the link to the earlier question.
If you don't have enough rep to vote to close, you can either skip it and let others handle it, or flag for moderator attention. If it's really a blatant duplicate, others will catch it soon enough. It's probably not worth bothering mods about something that basic that plenty of ordinary users can take care of for them.
Another option is to write a comment saying that you think this is a duplicate, and providing the link. That makes it really easy for those with higher rep to check out the supposed duplicate and vote to close if they feel it really is a dupe.