To be sure I was getting an accurate inventory, I cleared the needle list I had in Ravelry and started from zero. I went through all my needle cases. I gathered project bags. I listed each needle, even designating brands. I then created a separate spreadsheet for my circulars that I could color-code and manipulate to get a better handle on gaps and my needs.
I placed my order - a Christmas gift to myself - and purchased needles.
When the needles arrived, I pulled out my handy spreadsheet, filled in the newest additions and calculated how I could store them to make best use of my needle cases. I color coded my inventory spreadsheet and pulled out my handy label maker and went to work re-organizing my entire circular needle collection.
Approximately two hours later, very proud of myself, I had one complete, hyper organized circular collection, complete with laminated reference charts. I was never going to buy duplicates again, I was always going to know not only that I had the needle, but where to find it. I rewarded myself with a little knitting time on the sofa. I settled in, pulled out my project bag and immediately found two circular needles I'd missed, and recently replaced.
Spreadsheets are editable and there is always a knitting friend who can make use of your duplicate needles if you can't find another project to cast on.