It's just at first sight, they both just looks similar. In any case, here's the results of my little "investigation".
They both has a different sizes/shapes of boobs, location of moles (the one girl has them in places, where the other never had, and vice versa), and an unknown girl has more delicate facial features, at least for me.
All photos above was taken during one photoshoot session that took place on 28 June, 2015 (EXIF Info), so it seems that it's just a one-and-done work. Elina shoots for Slinky during several years.
Model's name are not mentioned in the description (or in comments) to any of the photos above. Unlike of Elina's name, that as i see not a secret at all.
And finally: you will not find any of the photos above in any of Elina's albums/galleries, including her own IG.
Here's another model:
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Also uncredited, and photos taken for just a one week before. Even more look alike for me, but I think that she isn't that model.