I hardly ever do! I Generally, if I've got one of something I'd rather get something different.
I do have the same Boden top in two different colours, though (the second one was mega-cheap in the sale). And I just bought a cardi on eBay for £1.40 because I recognised the style as being the same as one I already have and love.
And I have the same bra in three colours, but that doesn't really count.