What does Boxing Day have to do with boxing?
early 15c., "doubling," from Middle French duplicacion (13c.) and directly from Latin duplicationem (nominative duplicatio), noun of action from past participle stem of duplicare (see duplicate (adj.)). Meaning "repetition" is from 1580s.
duplication du·pli·ca·tion (dōō'plĭ-kā'shən, dyōō'-)
The existence or growth into two corresponding parts.