I don't tend to double up on the same products, and I'm not talking MAC lipsticks or eyeshadows here. I'm talking, literally, same product, in two shades. But there have been several products that I love so much, I just...had two. please don't throw rocks at me.
The first one isn't going to be a surprise. Probably my oldest makeup love, the Bobbi Brown Corrector (£19) has been a firm staple of mine. I always had mine in Peach until I started realising that it was just a tad too dark for me during my paler months, and would cast a shadow to my concealer - not exactly helping matters. So I immediately bought Light Peach without any hesitation, and I honestly wouldn't even hesitate to purchase a darker shade of this if I miraculously became really tanned some day. 
I don't ever double up on my bases. There are just too many options out there to have two shades of the same base product. But I've been in a strong committed relationship with the NARS Tinted Moisturiser (£29) in Grogenland for over a year now, even dedicated to make it work when I was too pale for this by mixing it in with my lightest bases. I finally decided to throw my silly rule of 'no two same bases' out the window and bought Alaska when the sales were going on some weeks ago. Let's face it, complete with SPF and all that jazz, this is the base that I reach for 85% of the time. I'm not quite sure why I have all my other foundations at this point. 
The next two don't exactly fit in the same category as the above mentioned products. But they still kinda do. I'm quite iffy about what products I spend my money on - especially if they're from the same brand, or the same line. So I wasn't particularly fussed about the Urban Decay Naked2 Basics (£23) when it came out as I was so done with the whole Naked line even before they announced the release of the Urban Decay Naked 3 a couple years ago. I had and was content with the Urban Decay Naked Basics (£23). But I had a weak moment and bought the latter version, fast forward to now, this is not one I regret. Do you need both? No. But the shades are different enough that I use both on an equal basis to justify having both. 
The same goes for the Jo Malone Fragrances (£42 ). I was content with my fragrance collection, and I was utterly in love with the Jo Malone White Jasmine and Mint Cologne as it carried some of my favourite memories. But then I sniffed it in Wood Sage and Sea Salt and it was pretty much just a matter of when, rather than if I would buy it. These two are different enough for me to not look back and weep at the money spent - and quite honestly, this is 66% of my regular fragrance wardrobe. 
I don't buy duplicates that often, but there have been certain loves that prompted the end result. Are you someone that buys the same product in different shades? If so, which are the ones that you have two of?  With Love,  Daphne x

Where to buy in Taiwan: selected Sogo Department Stores (Bobbi Brown, NARS, and Jo Malone), Beauty Bay (Urban Decay)