Here's what I dug up from my makeup stash for comparison - the much vaunted and raved about Rock and Republic Seduce, the one-with-the-fangirls MAC Flirt and Tease, and First Light Cosmetics Sundowner. All three of these were powder blushes, and they were in the same colour family. All three are plummy blushes with a hint of brown - a bit overwhelming to look at in the pan, perhaps, but lovely if applied with some caution on the skin. And yes, I do think fair girls can wear these colours too! (I'm NC20, and these happen to be some of my favourite shades.)
Anyway, enough with my gushing about plummy blushes. On with the swatches:
L-R: Rock and Republic Seduce, First Light Cosmetics Sundowner, MAC Flirt & Tease
Now, first off the bat, you'll notice that these are very similar. Very, very similar. They aren't 100% dupes for each other, but I guess once worn it's a little difficult to tell the difference. But there are some small differences.
The first small difference is that Sundowner is a lot more shimmery than either Seduce or Flirt and Tease. It doesn't have huge glitter bits, but it's more light-reflective than either the Rock and Republic or the MAC colour.
The second tiny difference is Sundowner is more plummy and rose in colour, while both Rock and Republic's Seduce and MAC's Flirt and Tease are more brown. But as you can see in the photo, the difference isn't very great.
The last difference is that of pigmentation. Rock and Republic had the best pigmentation, while MAC was the least pigmented. Now before I get lynched by all the MAC fangirls for even daring to suggest that MAC can be less pigmented than another brand, let me offer an olive branch by saying that Flirt and Tease is a Sheertone colour, anyway, so it isn't meant to be super-duper pigmented. It isn't sheer by any means, though, and you can definitely build it up to the level of kabuki clown cheeks, but it is a little sheerer than Seduce and Sundowner.
So, are all three blushes dupes for each other? Colour-wise, I think the Rock and Republic and the MAC blushes are more similar to each other than the First Light Cosmetics version. So if you want the closer dupe of Seduce, then Flirt and Tease is the one to get, even if it's a tad sheerer. But they're all similar enough that I can't tell you which one to pick definitively - it's going to depend on your brand preference (high or low end?), budget (cheap or expensive?), and other preferences in terms of form (loose powder blush or pressed powder blush?), pigmentation (more pigmented, or less?), and colour (more red, or more brown?).