Restore the DISTINCT on ListElementChildren

Refs requests#727

A long analysis has shown that a commit from a year ago removed a .distinct(), which can cause duplicate elements when their parent has multiple parents.

Please copy the arkindex.documents.tests.test_element_paths_api.TestElementsAPI.test_table unit test to test_children_elements, and replace the assertCountEqual and assertListEqual near the end of test with assertions made on the results of the relevant calls to ListElementChildren; this will test both for this bug and for #769.