Each drip of hot glass that hits the shaping roller on a marble production line is called an "ingot". A "double ingot" is a marble error that occurs when two marbles meet on the roller and fuse together into a single, shaky sphere.
Because they tended to be either thrown out or taken home by production line workers, double ingots (as well as triple and even quadruple ingots) are rare collectibles. They are often found and sold by marble diggers (those who visit abandoned marble factory sites and dig for discarded marbles around the area), though more modern marble companies like Jabo and D.A.S. have purposefully distributed their double ingots.
The first two marbles in this set are from Akro Agate - lifesaver and sparkler respectively - while the last three are from D.A.S. runs - carnival and wildflower respectively.