Here’s the excerpt from MIDI 1.0 specification, page 62 (A-4), regarding multiple Note On messages. Looks like I remembered it incorrectly, duplicate Note On messages are allowed, as long as there’s an equal number of Note Off messages with matching Note number.
Looking and GrandMA2, it might not care about there being not enough Note Off messages, but since 1.11.5 firmware of Midihub, it filters out the additional Note On messages without the Note Off message in between at the physical output boundary. Now I’m thinking whether we should make this configurable per-port. 
Anyway, to produce the required number of Note Off messages, either the Note Length pipe or another Transform pipe could be placed to take care of producing the appropriate Note Off events.