Do the items "Collected Stories: Twenty-one stories" and "Twenty-one Stories" contain different sets of stories? If so, they should be separated. If they contain the same set of stories, they remain combined.
James wrote: "There is an actual separate edition of Twenty-One Stories here. Is the best course of action to separate all of the incorrectly-filed Twenty-One Stories and then combine them with that one?"
Assuming that "Collected Stories: Twenty-one stories" contains different content, then yes.
Also, I noted that "Complete stories" is also combined with "Collected Stories". According to the description, it was "Previously published in two volumes Collected Short Stories and The Last Word and Other Stories". Therefore, Complete Stories should also be separated from "Collected Stories".
Make sure you leave librarian notes, using the link at the top of the edit page, to explain why these separations are being done. Otherwise you may end separating these again later.
I checked the two I saw on Worldcat.org. It seems worldcat doesn't list the subtitle either, so I would say go ahead and remove it, especially for this case where it seems to be creating confusion.
James wrote: "There is an actual separate edition of Twenty-One Stories here. Is the best course of action to separate all of the incorrectly-filed Twenty-One Stories and then combine them with that one?"
Assuming that "Collected Stories: Twenty-one stories" contains different content, then yes.
Also, I noted that "Complete stories" is also combined with "Collected Stories". According to the description, it was "Previously published in two volumes Collected Short Stories and The Last Word and Other Stories". Therefore, Complete Stories should also be separated from "Collected Stories".
Make sure you leave librarian notes, using the link at the top of the edit page, to explain why these separations are being done. Otherwise you may end separating these again later.