First of all, I don't know what I was thinking buying DL nail polish. I bought Mermaid's Dream (£18) first, followed by the "Get this party started collection" for £28 (for 3 bottles), so it was quite expensive. I will do a review with swatches of the full set soon and share my thoughts. It was an impulse buy and I don't think I will spend this amount of money on nail polish ever again.
After seeing many dupes for "Happy Birthday" I decided to finally buy one, so I can show you the difference. I got the one from Technic in a shade called Carnival. I paid £2.07 for it including delivery (I got it from Amazon but it seems to be sold out now). I need to admit, they look very very similar. They are both multicoulour glitter nail polishes and that's enough for me to call them dupes ;) Happy Birthday has got more green and orange glitters and Carnival contains more blues and pinks. In the bottle they look different, but once applied they look very similar. Another thing I need to mention is the smell. Carnival smells really really strong and very unpleasant and gave me headache. DL Happy Birthday's smell isn't as strong and is manageable.
EDIT: I seen a few pictures of the Technic nail polish on other blogs and it looks a little bit different, I think that it depends on the batch, as I don't think anybody would attempt to make a fake of a £1 nail polish.