An ingot is a measure of glass equal to the size (mass) of the marble intended to be produced. It is normally sheared and put on rollers to impart the round shape. A "double ingot" occurs when (usually) two separate measures of glass intended to form two separate marble stick together and get rolled into one. The result usually has a clearly defined hemisphere where they came together. Almost all of these came from factory digs.
Here are a two examples of double ingots: