Code
T5008 heavy-work-note
A note: work that may be done twice. The check finds it, and refuses nothing.
Details
An action that states worker or background is handed to a machine, which may be lost while it works: its work is then given again from its start. What it sent out before - a mail, a call of a service - may be sent twice. An action that states sends: false says that it sends nothing out, and is not noted.
A batch that states split cuts a batch that failed in two until the one entry that fails stands alone: the good entries are carried out again on the way.
The fix
State sends: false where the work sends nothing out. Where it does, give each call it makes outside a once-only key, so that a second making is answered the first answer.