"What item do you purchase that always inspires you to buy a 2nd item in other words, what item can you just not buy one of?
Over the past couple of years, when I've purchased a Jolees Disney-themed sticker, I've bought two of the same at one time. I did that because I'm putting together a scrapbook for each of my boys and we have some really happy memories of our time at Disneyland that I wanted to scrap.
I used to buy my ATG tape in bulk (12 at a time!), but now that Michael's carries it, it's cheaper to just go there and use a coupon and buy one box with two rolls.
I bought my Copics in multiples, but I don't think that really counts as not being able to resist just one. I bought them in bulk, went in on an order with friends to get free shipping and a discounted price. Most of my Copics cost me less than $4.