Comparison!

Alright people! Look what I have!

Milani - Gems FX and Deborah Lippmann - Happy Birthday

Milani - Gems FX 

Deborah Lippmann - Happy Birthday

As you can see, they're NOT dupes. Milani doesn't have pink or baby blue glitter. Their blue glitter is a little darker. Milani's glitter also has more of a same color family feel while DL jumps around with true red, baby blue, dark blue, gold-ish yellow, green, and fuschia pink glitter.

Another major difference is that DL uses SQUARE little glitters while Milani uses CIRCLE little glitters.

Happy Birthday on the left, Gems on the right
Happy Birthday has more of a chaotic confetti feel, while Gems feels more of an organized chaos. Does that make any sense?

I would suggest getting Gems if you're really lemming Happy Birthday.
Gems is a lovely glitter polish that will get the job done.