To tea duel you need each participant to have a cup of tea (although I suspect hot chocolate could be substituted) and some tea biscuits or digestive biscuits. I’ve been advised not to use shortbread as it is surprising sturdy. Two competitors are seated across from each other. 6 cookies are placed on a plate between them. The each select the cookie they believe to be the sturdiest and position it above their tea. On “dunk” they must dunk at least half their cookie into the tea for a 5 count. Then they remove it. The winner is the last person to eat their cookie without the cookie breaking.
More complete rules can be found at The Honourable Association of Tea Duellists