Because I spend so much time perusing the patterns online, I have a hard time remembered what I have and have not purchased. If I have sewn a pattern, especially for myself, I can usually recall, but that is not always the case for something I haven't made yet. As such, I've ended up with a couple of duplicates in my collection.
Knowing something had to be done, I turned to Google Documents. Any sort of records for my patterns had to be accessible from my computers and my phone, so their spreadsheets were ideal for this task. I created six: Butterick, McCall's, New Look, Simplicity, Patterns to Purchase, and Other (which includes Burda, Laughing Moon, Vogue, etc). Each has four columns: pattern number, description, the fabric I made it in (blank if I hadn't sewn it yet), and notes.
While this makes me feel a little anal retentive, I feel accomplished that I managed to organize them and there will hopefully be no more mistakes.