Are you grateful that they linked to the other thread for you, so you don't have to? Or are you a bit annoyed that someone else is doing your job for you?
I generally find it useful when a member has already provided a link, if I'm responding to a report about a duplicate thread. Or if I notice a dupe thread and someone's already taken the time to provide a link to the other thread.
Sometimes someone will send in a report about a duplicate thread in a discussion area I don't spend a lot of time in generally, and won't provide a link; and then I have to go through page-by-page looking for the original thread. Having a link to check on usually saves me a lot of leg-work in cases like that.
The one caveat being that occasionally we have had members that can get a bit carried away with things like that. Generally, we like to see members being helpful. So if someone politely posts a dupe thread link, then that's a good thing. Being snarky or rude about it, however, is something we tend to frown upon.