I've seen a few wording variations on this and hope this contributes to the conversation:

https://github.com/odoo/odoo/issues/152695

Steps to reproduce:

- Create a marketing automation to send an email on new opportunities

- Do not set Unicity

- Add sub-task to add tag "Email Sent"

- Create new opportunity

- Wait for cron job to run

- View Participants:  Workflow task (send email) is "PROCESSED"

- Email is received

- Add an opprotunity with the same email address again

- Wait for cron job to run

- View Participants:  Workflow task(send emal) is "CANCELLED - Email cancelled"

- Email not received

Current behavior:

The workflow task is cancelled yet subsequent tasks are still processed, i.e. add tag, etc.)

Expected behavior:

If the workflow task is CANCELLED, no other tasks should run under it.  There is no indication in the workflow about any cancelled tasks

Workflow should show the task was cancelled; no other tasks under it should complete.

Since Unicity is not set, the sending to the same email address should successfully run again.