I went through all my mac eyeshadows, and this is what i found:
Brun, Dark edge (from tempting quad), Concrete and Mystery are extremely similar to eachother. I wish i knew this earlier, because i basically have 4 almost identical colors.
Rich flesh is very similar to both Cork and Wedge. Its like if Cork and Wedge had a baby it would be Rich flesh. But they are all so similar that you really only need one of them imo.
Haux and Quarry is very similar. Haux is a tiny bit darker. Maybe a TINY bit more red if you really study it. All in all you dont need both.
Mulch, Bronze and Romp are pretty similar. Romp is a bit more gold. Mulch is a bit more plum. I wouldnt have gotten all of them if i knew how similar they were.
Graphology is like a matte and a tiny bit darker version of Contrast.
Seedling and Omega are identical in the pan, but the texture is different, and swatched on the skin Seedling is a BIT more gray/taupey. But you definately dont need both.
Handwritten and Embark are pretty much identical colors. But because Handwritten is a matte2 it has slightly better texture imo.
Hey is very similar to Naked lunch, but creamier texture. Its also very similar to All That Glitters, but ATG is slightly more peach.
Brule and Blanc type are very similar. But Blanc Type is a TINY bit lighter and has better payoff. (matte2)