I have in the past bought something I already had. To keep from doing this, I started a little notebook (well, actually it's one of those dollar photo albums). In it I have every tool (punches, dies, scissors, templates, etc.) shown by picture or listed. This has helped with not duplicating anything. Like right now I'm buying nesting circles and square punches. I have punched what I already have and its measurement on one page.

I have another notebook that's regular size. Because I host a monthly crop at church, the large notebook not only shows what I have but where it is located. It also has Sizzix die cuts by color. For example, the teapot is cut out of red paper and the tea cup is cut from green. And by looking in this notebook, you'll know exactly how big each is. For Sizzlits, I just used the same color (scrap)paper for a set with its name underneath. Since they are stored alphabetically it's much easier even for me to see exactly what I have and where they are.

Here's a link that shows what I'm trying to explain.
http://www.scrapbook.com/scrapbook_layouts/showphoto.php/photo/118728/cat/605/page/1