There is a setting for “Author Name” under “Citations.”
“Omit repeated authors (e.g. Smith 1999, 2000)” is checked under “Consecutive Citations by the Same Author.”
This setting only works as advertised if all the authors for each paper are exactly the same – so Smith, Jones, Frank, and Works 1999 and Smith, Frank, Jones and Works 2000 are still cited as (Smith et al 1999, Smith et al 2000) rather than the preferred Smith et al 1999, 2000).
There is no reason for this repetition of identical information, it does not help distinguish the two citations in the list to duplicate the information, and this Endnote behaviour should be corrected.