Comment 7 for bug 378333

the way you describe would work ideally, the issue is that we got so many bugs and duplicate that we would either spend the week looking for duplicates numbers or keep hundred of duplicates open and make the bug list un-usable for people who work on bugs which would defeat the purpose of bug reporting.
The current way is a compromise between those, we close bugs that we know to be duplicates and let the user search for the corresponding open bug.