First off, Julep Love, which I think was included in every box this month, is pretty damn close to Sinful Colors Gilded. Here's one coat of each over black:
Putting them over black shows that they're not completely identical: Love has more gold shimmer (although that may be due to me always getting too much polish on the brush thanks to Julep's stupidly long bottles), while Gilded has more red glitter and its gold shimmer is more yellow. I really don't think you need to own both of these.
Second, here's two coats of Julep Glam Roc (from the It Girl box) and Forever 21 Black Glam (even the names are similar!):
These are both blackened purple textured polishes with magenta and gold sparks. Glam Roc seems very slightly darker (the difference is less obvious in real life), but the glitter seems to be the same. I also prefer the texture of the Forever 21 polish--I think it's packed more tightly.
So I'm a little bummed that I paid for two polishes that I had cheaper dupes of. Obviously it's too late for those who got the box this month, but if you were thinking of buying either of these Juleps on sale later, get the cheaper brands instead!