What happened to the option "mark as duplicate" in the edit question dropdown? I no longer see it.
That feature has been removed.
The Mark as duplicate option is no longer visible in the Edit question drop-down menu because that feature has been removed for regular contributors.
As you know, that feature allowed you to signal duplicate questions by sending a "merge request" to the team.
As it turns out, not that many contributors used that feature, to put it mildly, so the team decided to remove it completely.
Now you can let us know that a question is a duplicate by @mentioning one of the moderators / heroes, and we will go ahead and merge the questions.
So, to sum this up, regular contributors no longer have the option of marking questions as duplicate by using that feature. However, you can and should give us a shout when you spot one.
Sadly enough, we have plenty of duplicate questions floating around, so any help we can get will be greatly appreciated.