Previously if a job was listed more than once for a project, it was
ignored. That's pretty arbitrary (it silently dropped the second
without an error; and if there are two, which is the right one
anyway?). OTOH, it's potentially useful to run a job more than
once in order to increase the chance of encountering
notdeterministic behavior. And if listing a job twice is an error,
it is now more likely to be noticed by the operator.
This removes the check for duplicate invocations of a job.
Change-Id: If8e2e8cc3fca855bd6b14eb3a957dadddfe143ed
3aa3727225