As Stack Overflow grows — or any other Q&A site in the Stack Exchange network, really — there’s a natural pressure to discover and link duplicate questions. The more questions you have, the higher the possibility a given new question isn’t in fact a new question, but a duplicate of an older existing question. Because of this, we’ve continually enhanced the tools for finding, linking, and merging duplicate questions:

One thing I want to be clear about, though, is that duplication is not necessarily bad. Quite the contrary — some duplication is desirable.  …

The ability to cast a question deletion vote against a closed question is earned at 10,000 reputation. It was always intended as a tool for getting rid of useless, bottom-of-the barrel type closed question content. But lately we’ve had to deal with a rash of question deletions, some of which I felt destroyed useful answers. To address this issue, we instituted scaled deletion votes. Question deletion still requires at minimum 3 votes from 10k rep users to achieve a delete (or undelete) — but the more votes a question and its answer have, the more delete votes required to delete it. This is really more of a safety measure than a complete solution, though. …