Execute task via form generated cron

(Frenchy sorry)

Hi =)

I need to generate some crons via a form, if the form is posted at
19h49, the task must be executed every 20minutes from 19h49 and n
times (n depends of a form field value).

AND!

It could happen that some forms posted by differents users generate
the task at the same time with differents arguments(the user id). In
this case i need to "verify" if an equivalent task is programmed
before executing it, once.

Is that possible?

Thanks,
Maz.