Multiple "copies" of audiobooks in iTunes
Through trial and error, found the source of the problem and a solution that consistently works for me. Explanation and screenshot follows.

Source of the Problem

The Sorting tab has three groups you can sort on: title, chapter, author. If the text in the specific field is disabled, then there will be duplicate copies of a given audiobook.

Note: The words "disabled" (for gray text) and "enabled" (for black text) in this context are my own and may or may not be the same terms used by Apple devos.

Solution

Simply retype the disabled (gray) text.

The screenshot shows an example where the "chapter" field was disabled. When I retyped "Mixed," the text changed to black. Clicked OK, and the duplicate audiobooks disappeared.

The same solution worked for me when title or author were disabled. Note that the "sort as" field is ok in gray.