Its occured to me that duplicate questions usually (obviously) ask a question in a different way, and are not deleted to provide googability. This being said, isn't it in our best interest to encourage duplicates in this manner as all they need is a simple close and we are better off for their excistence?
They're not a bad thing, but I don't see why we should be intentionally seeding our site with questions only asked to be closed. These questions will come naturally anyway.
On a whole, are duplicates a bad thing? No. Good duplicates can happen, and I'll take one of my questions as an example. I did a search for this question, didn't come up with anything, and then asked the same question but in a completely different way from the original, but we were asking the same thing. As you say, this improves searchability, and to this day my question has more views than the original because it's more searchable (and has a good answer to boot).
Should we get our users to ask a question which they know will be closed? Never! What's the point in that? I take this standpoint not from the closed as duplicate reason, but from all close reasons. If you know your question is going to be closed, for any reason, don't ask it! As @badp said, these questions will come along naturally because the original question was asked in a different way, or because someone's google-fu failed them.