Filters manipulate sets of titles in which no title may appear more than once. Furthermore, they often need to append one such set to another.
This is done in such a way that, if a title would be duplicated, the earlier copy of that title is discarded. The titles being appended are dominant.
For example, if a selection contains Andrew Becky Clara Daniel and Andrew Barney Clara Daisy is then appended to it, the result is Becky Daniel Andrew Barney Clara Daisy.