Don't treat attempts to read an already read file list as fatal errors.

Don't treat attempts to read an already read file list as fatal errors.

* src/names.c (add_file_id): Report error and return 1 if the file
has already been read.
(read_next_name): Don't try to open file if add_file_id returns 1.
(name_next_elt): Simplify conditional.