Not movies, but I have bought my mom CDs before to fill missing holes in her collection of certain artiss before (like Reba or Tim McGraw). Those artists have around 10 to 20 albums each, and she's only missing a couple so I buy them for her.
I feel like I am a completist in bizarre ways--to the point I'm bothered when I can't have all of something or everything the "right" way. For example, I have all of the Series of Unfortunate Events books, but only one out of the 13 has a cover that's plastic and removable. And I have all the Chronicles of Prydain books with the beautiful illustrations by Evaline Ness‎ except for the last one. Whenever I look at the ones that stand apart, it gets on my nerves, to the point that I want to re-buy them.

But I have to remind myself not to waste money.