Hmm double bubble. I only think of one thing when I hear it: double bubble bubble gum. Aside from being my favorite gum for bubble blowing I have found this gum is easy to create a bubble within a bubble with hence the name " double bubble".
Aside from that a double bubble would be a bubble made of two I suppose.
Bubble gum? That's the first thought that comes to mind.
According to google however, (amongst a variety of choices)double bubble also refers to the 1950s coupe.It is also an airplane designed by the researchers at the University of Massachussetts Institute of technology which could cut aviation industry fuel use by 70%. This was reported in 2010 by the Guardian Environment Network. Hope this helps