The following chorales in the Riemenschneider edition are duplicates or near duplicates of at least one other in the set: 5, 9, 23, 53, 64, 86, 88, 91, 93, 100, 120, 125, 126, 131, 144, 148, 156, 177, 178, 195, 198, 199, 201, 235, 236, 248, 254, 256, 257, 259, 282, 295, 302, 305, 306, 307, 308, 309, 313, 318, 319, 326, 328, 349, 353, 354, and 361. Therefore, in the "371", there are in fact only 347 discrete chorales. Several of the duplicates appear in different keys, and three of the chorales (125, 328 & 319) also contain slight alterations. Only in the case of Chorale #125 do the alterations amount to more than a note or two of figuration.
So how are these duplicates explained? The question should really be separated into two: First, why did the editors of the Breitkopf collection fail to recognize and eliminate duplicates? Second, how did the duplicates end up in the collection in the first place?
The first question assumes that the inclusion of the duplicate settings was indeed an error—that is, an oversight on the part of an editor. This assumption is supported to a degree by the fact that one duplicate setting in the Birnstiel collection, published in the 1760s, was eliminated by editors of the Breitkopf collection, published two decades later, when the former served as a basis for the first volumes of the latter. However, the fact that four other duplicates in the Birnstiel were not eliminated in the Breitkopf would seem to indicate true negligence on the part of the Breitkopf editors. (These four are Breitkopf 23=88, 52=178, 100=126, and 148=177.) Gerd Wachowski has suggested that the reason these duplicates were not detected by editors is likely due to the fact that different headings appear for the identical settings (Bach–Jahrbuch, Vol. 69, 1983). This is true of three of the four Birnsteil duplicates that were not eliminated, and it is true for many of the duplicates appearing in the later portions of the Breitkopf. The explanation for these differing headings used for the same chorale tune lies in the fact that the mixing of texts and tunes was common in the Lutheran chorale tradition. An additional consideration is that four of these duplicate pairs appear in different keys making detection more difficult. Still, the number of duplicate settings that are virtually identical in the Breitkopf shows a genuine lack of care by the editors.
But how did these duplicates end up in the Breitkopf collection in the first place? While we know very little about which sources were used when the collection was first compiled, it is fairly certain that multiple sources were used. This can be deduced from certain loose patterns that emerge in the arrangement of the chorales. To point out one example, the sequence of chorales beginning with Chorale #248 in the Breitkopf roughly corresponds to the beginning sequence of the earlier Dietel collection from the 1730s, a fact that strongly suggests that either the Dietel collection itself was used as a source, or the same source was used for both. The number of duplicates in the segment of the Breitkopf beginning around #301 also suggests that a new source which contained some of the same settings as a different source used earlier in the collection may have been used.