For me finding a rare poster at a great price is always a thrill, so when I do stumble across a title I have that I know is at a steal of a price I'll usually buy it. I find it partly satisfies me as a collector and partly satisfies me from a potential future re-sale. I try not to go too crazy; I'd have doubles of maybe 5% of my posters.
For all the married collectors out there, you'll understand that it is worth the investment in duplicate titles just in case the wife decides one day that "we" should sell of some of "our" collection!