I think that for community sake it would be nice if we would have a flag for duplicate forum posts (similar to Stack Overflow) but with additional mandatory input, link to original post.
Current available flags are not suitable for that, only Something Else would fall to that category.
I think as it is now, it would indeed be the way if you flag duplicates as Something Else with a link to the duplicated topic(s) in the input box. We will take it from there.
This doesn’t mean we wouldn’t want it (it would still mostly simplify the work of the moderator, as you have already spent time to find the link and paste it, be it now or in a new flag type), but this depends if meta.discourse.org chooses to add it one day