By default not-selected
If KojiSCM requires workspace, then the single exact job can not be run in parallel
If KojiSCM don't requires workspace, then the single exact job can be run in parallel
if you unselect it (so it don't require workspace), you may run the same job in parallel
In that case, be sure, that next pooling is invoked after the checkout is finished, otherwise you end with same build in queue
General usecase is:

some long job is running, but otherwise the host is bored
everything else is done except next this job
in desired project select "Execute concurrent builds if necessary"
unselect this
schedule/let schedule another job and wait until it is in execution (in parallel)
select this
in desired project unselect "Execute concurrent builds if necessary"
after its checkout is done, verify processed.txt in job's folder