I think Paul makes a good point. My first reaction is that there are a great many certified loops that are used in races of a variety of lengths-- especially, for example, for race walks, but also for low-key club events. The key thing may be this question: is the course altered or disrupted for any of the runners by the fact that some runners are doing more or fewer loops? If it's like a track, there is no particular disruption if some runners pull over to finish at 5 miles while others go on to complete 10-- just put the finish chute on the outside lanes of the track.
But things may have changed since the old days of using loops for ultrarunners and walkers, etc, with more races using some form of electronic timing. Nowadays there seems to be more concern about how close continuing runners come to the actual finish line-- and also concern about runners who have finished a shorter race getting "read" as finishing again.
It may be that multiple loop races in many cases need to be laid out with a repeatable loop with an "early pull-off" spur for a finish line. This spur can be used after any number of loops, as long as the distance to the finish will be equivalent to completing a loop.