Is there a way to report that a question is a duplicate of another one?
Some users are able to cast a close vote, but what can users who see a duplicate question but cannot vote to close it do?
Nowadays*, the best option is to flag the question as a duplicate. You only need 15 reputation for this. This flag will leave a comment (even if you do not have the reputation to comment everywhere, which on most sites comes with 50 reputation), just like a normal duplicate close vote would, and push the question into the Close Votes review queue.
Just leaving a comment like ire_and_curses' answer says won't put it into the review queue; Jeff's answer might, depending on the site, get you a declined flag because "flags should only be used to make moderators aware of content that requires their intervention"; duplicates can be handled by the community.
*: this was not an option at the time this question and the other answers were written.