The accepted answer to this question says that when you click "flag" on a question, there is an option called "it is a duplicate". When I click "flag", though, I only see the following options:
- it is spam
- it is offensive, abusive, or hate speech
- it is very low quality [Note: this option doesn't appear on meta sites]
- other (needs ♦ moderator attention)
After some additional troubleshooting, I have discovered that I do see the "duplicate" option when I try to flag questions on Meta Stack Overflow. However, I do not see the option on Stack Overflow, Ask Ubuntu, or Meta Ask Ubuntu. I just received the flag privilege on Stack Overflow today, but I have had it for several days on Ask Ubuntu and Meta Ask Ubuntu.