The Library Association rule is: "35. The heading is not to be repeated; a single indent or dash indicates the omission of the preceding heading or title."
The Index Society rule is rather fuller: "17. A dash, instead of an indentation, to be used as a mark of repetition. The dash to be kept for entries exactly similar, and the word to be repeated when the second differs in any way from the first. The proper name to be repeated when that of a different person. In the case of joint authors the Christian names or initials of the first, whose surname is arranged in the alphabet, to be in parentheses, but the Christian names of the second to be in the natural order, as Smith (John) and Alexander Brown, not Smith (John) and Brown (Alexander)."
The reason for the last direction is that the Christian name is only brought back in order to make the alphabetical position of the surname clear; and as this is not necessary in respect to the second person, the names should remain in their natural order.
Dashes should be of a uniform length, and that length should not be too great. It is a great mistake to suppose that the dash is to be the length of the line which is not repeated. If it is necessary to mark the repetition of a portion of the title as well as the author, this should be indicated by another dash, and not by the elongation of the former one; thus:—
Milton (John), Works in Verse and Prose, Printed from the Original Editions, with Life by the Rev. John Mitford. 8 vols. 8vo. London, 1851.
—— Poetical Works, with Notes, Life, etc., by the Rev. H. J. Todd. 6 vols. 8vo. London, 1801.
—— —— —— Second Edition. 7 vols. royal 8vo. London, 1809.
—— —— with Notes, edited by Sir Egerton Brydges. 6 vols. small 8vo. London, 1853.
All the dashes except the first, which represents the author's name, can be got rid of by using the words [the same] or [another edition], etc.