Merging questions certainly has happened in the past; that would happen only when you had two separate questions, both of which had very good answer(s) worth saving, which were determined to be sufficiently similar to not merit two separate questions. I'm fairly sure it's quite rare (in particular, a mod would have to do it; there's no 'merge' flag for a regular user, so it would have to come from an 'other' flag or a request from meta).
In general, the reason for the duplicate redirect (as opposed to deletion) is specifically because of what you say: perhaps the duplicate might be more findable, even if the original has answers (or better answers). Questions closed for Off Topic or other reasons are often deleted eventually; questions closed for Duplicate are not, so that Google/etc. index them and will bring them up in results if they are indeed well written questions.
The "is any effort used" of course varies by question... it takes 5 votes to close something of 3k+ rep users, so hopefully at least one of those users went to some effort. I would suggest that almost all of the time that is the case, but certainly occasional questions might get 4 robo-reviewers and one initial misguided soul.