Allow duplicate backend registration when event has "allow same participant emails"

We have the following use-case:

• Organisation using CiviEvent for a box office, i.e. to sell tickets for concerts
• People may buy multiple tickets for concerts, and often not at the same time.

For example, they may have bought a ticket during pre-sales, then later bought a ticket as a gift for a friend.

The front-end allows this, thanks for the "allow same participant emails". However the backend does not validate this option. When adding a backend registration, if the participant has already bought a ticket, the admin will get an error message: "This contact has already been assigned to this event."

Looking at Stack Exchange, I found this thread:
https://civicrm.stackexchange.com/questions/15826/enable-multiple-event-registration-for-same-purchaser-participant

Since we use the Event Cart and the boxoffice extension, I implemented a workaround here:
extensions/boxoffice@526703f4

but we could avoid all those overrides by adding those 4 lines of code to CRM_Event_Form_Participant.

cc @cmtool