This is essentially the same error reported in the Open Discussion forum, with the same title. It has been simplified, so that there are fewer files and objects.
This test case makes a capital letter "A"
In the file: fontTstA.g, combinations fontET-A.c and fontET-A0r.c differ by one ars: fontET-A0.L.s and fontET-A0r.L.s, respectively. These two ars, in fact, produce the exact same object. The only difference is that the order of the points are reversed, hence the "r" designation.
One perhaps peculiar thing, is that the *.L.s ars are topological tori, i.e., the first and last curves are identical. This has nothing to do with the problem, though. The test case in the Open Discussion forum, was done without topological tori, but it fails the same.