I have plenty of dups. With the wild free for all collecting I have been doing, (7,000 in 11 months) I have dups. I would guess at around 3% dups or less though (I have a very good memory) Mostly unwanted dups in the Rock area, but a dozen or so on pupose in Jazz. I have 2 6eye Mono Sketches of Spain, and a 6eye stereo. 3 sets of the Benny Goodman's Carnagie Hall 2 are the pre 6 eye green Col and one set a six-eye. I LOVE Stravinsky, so I have some dups of his stuff, but otherwise I am pretty good on the classical. Though I do collect all the Beethoven Piano Sonatas by different artists I can find. Ditto Gershwins pop pair, Rhapsody in Blue and American in Paris.
So Rock is the one area that I am sort of not often certain if I have something... I know I have a several "The Cars'... each has dups, I am certain. And other Rock bands. I have 3 sets of Allman Bros At Fillmore East.
Some dups I buy because of great condition/price As the mint Harold Budd/Brian Eno 'Pearl' dup I just bought for $0.18
Most of the stuff I have still needs to be sorted out. I have 40 x 120 in each, boxes of stuff I bought and haven't had the time to play, The most I play a day is 10 LPs. and I buy about 20 a day, average. I am buying to set myself a good stash when I retire. (I am 54) because now is the time to buy in my area. The oceans of cheap, excellent LPs for sale in my area, can't last forever.