Two airplanes have equal weight and equal wing area. One is a biplane. The biplane has 4 wingtips that cause tip losses. The monoplane has only 2 wingtips causing tip losses. To simplify the example, lets assume that the tiploss for a tip on the biplane wing is equal to the tiploss for a tip on the monoplane wing. In fact, they will be different, but not by very much. So if we compare the two airplanes, the biplane is suffering the effect of 4 tips versus the monoplane's 2.

That's one major reason that for best efficiency, monoplanes beat biplanes when the wing areas are the same.