I have to try to rationalise my Terry Pratchett collection. Apart from a few distinctive titles, I can never remember which ones I have read. I do find them occasionally in charity shops and second-hand bookshops, where the price can encourage me to take the risk of buying something I've already got. A few duplicates have inevitably crept in.
You would think making a list would help. For completists, here's Fantastic Fiction's page for Terry Pratchett -
I have never found one I didn't enjoy. Happy reading!