Task API - Handling Already Finished Task

I'm making an API and have a function which takes a task and runs it. When the task is finished successfully, it's status is set to 'Completed'. Now, lets say the user of the API accidentally (or for whatever reason) sends that same task (or 开发者_如何学JAVAany other already completed task) back into the same function. What should the API do?

• Throw an exception
• Pretend as if I've rerun the task and tell the user (through events or whatever) that it is done/completed (again).
• Do nothing and just ignore it.

Is there a standard or best practice for something like this?

Pretending to rerun hides what's probably a user error - this can lead to deadlocks or other logic bugs (i.e. - I create an event, wait on it and run a task that should reset it at some point - it never happens, deadlock). Also done handlers may fail if invoked twice per one successful task run.

Doing nothing is more or less the same - done handlers can't fail now :), but they are not invoked at all - a bug is more probable if done handler performed necessary communication with the spawning thread.

The worst thing is - these may happen or not happen, depending on the timing. I.e. the task may still be running by the time the user calls the function the second time (what do you do then, by the way?)

So, do throw an exception unless task status is "not started". The user can always check the status and perform the necessary processing in the unlikely case she needs it.