In the latest commit only the main hand is checked for ingredients when clicking the cauldron.
This sometimes causes two items to be added to the cauldron at once.

The interact event is called twice!! once for each hand, which is extremely unintuitive and should never happen but seems to be intended: https://hub.spigotmc.org/jira/browse/SPIGOT-1570
Also when holding certain items it is cancelled, but just for one hand !?

Needs a workaround to always check both hands but still only listen to non cancelled events

Fixed, both hands can now be used to add items to the cauldron. Some items like grass cause the off hand to be ignored though. Test snapshot here:
brewery-1.4.5-SNAPSHOT.zip