Documents listed in HTX catalogue files are added into the documentation set after all hypertext documents have first been found using the HTX_PATH search path. If a document is found in hypertext form, it occludes any subsequence occurrence of a document with the same name in a catalogue file. This means that if you convert an "old" document into hypertext form (with a ".htx" file extension), the new version will automatically be found in preference to the old one -- there is no need to remove it from the catalogue file.
Catalogue files are also found by following the HTX_PATH search path after it has been used to find hypertext documents, and are recognised by the name "htx.catalogue". If more than one catalogue file is found, their contents are simply concatenated in the order in which they are found.
Duplicate entries for a document are permitted in catalogue files (and can also arise when catalogue files are concatenated). They provide a mechanism for a document to have alternative titles. This can sometimes improve the usefulness of document searches if the original title lacks any useful keywords (you might think of this as combining both a title index and a subject index into the same file). If more than one title entry is matched for a particular document, then the one that occurs first in the catalogue file(s) is used.