here's something else that may (or may not) be related:

A term used in various senses in the game of marbles (see quots.).

1823 E. MOOR Suffolk Words s.v., A player knocking two out of the ring cries ‘dubs!’ to authorize his claim to both. 1882 M. H. FOOTE Led-Horse Claim iv. 62 ‘What is it the boys say when they play marbles?’..‘Fend dubs?’ Hilgard suggested. 1896 Dialect Notes I. 220 In Missouri..dubs means, not doublets, but that the player has blundered, and by crying ‘dubs’ is entitled to play again.