Yep. I own two copies of the 1e Player's Handbook and Dungeon Master's Guide, five copies of the 2E Player's Handbook, two copies of the 2E Dungeon Master's Guide, two copies of each of the 3E and 3.5 Player's Handbook, and several duplicates of published adventures (I6 Ravenloft, Tomb of Horrors, et cetera).
I've never went out of my way to get duplicates or even paid for them--people have given them to me when they quit the hobby or found them in storage. I'm not adverse to purchasing duplicates, however... If the price is reasonable (say, around $10) and the book in question is in better shape than my current copy. I'd love a new copy of 1E's Unearthed Arcana, for example.