Dubious placement of Frankfurt on the included map.
Ran into a lot of examples like that when merging two existing georeferenced datasets of plague outbreaks in Europe. Had to do quite some manual checking, and made good use of the damerau-levenshtein distance between two strings to recognize alternative spellings.
end-result here: https://figshare.com/articles/plague_outbreaks_in_medieval_E...
In this particular case, it's Frankfurt (Main) vs Frankfurt (Oder).
I'm a little puzzled as to the inclusion of Frankfurt (Oder) on the map. It was a city of decent size in the Middle Ages, but nowhere near the importance of any of the other cities displayed on the map.
For example, Givry is a tiny place somewhere in France, but for which by happenstance the parish registers at the time of the Black Death survived. They record the daily number of deaths, which end abruptly when presumably the priest died after recording ~620 deaths since the beginning of the epidemic in that town. The town's population is estimated at around 1170 people, based on the death rate in the earlier months, leading to a lower end estimate of 53% mortality (if the death of the priest was also the end of the epidemic).
(from page 306 of http://www.persee.fr/doc/bec_0373-6237_1939_num_100_1_449196)
[0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 2 0 0 0 0 0 3 2 0 0] ;; jul. . approximate start of epidemic somewhere during jul.
[0 4 4 1 0 2 1 7 5 3 1 1 4 5 3 3 4 1 5 2 2 4 3 6 2 2 2 6 3 8 4 6 10] ;; aug.
[6 9 8 8 4 6 15 3 11 24 7 10 15 10 14 11 17 8 6 3 17 6 11 16 7 16 12 5 10 7] ;; sept
[7 6 7 7 8 7 6 6 9 4 7 9 3 14 5 5 4 8 6 7 3 2 5 4 4 2 3 1 3 0 6] ;; oct
[3 4 3 4 0 2 1 0 4 3 2 1 0 0 5 0 0 0 3] ;; nov, records end with the dead of the priest, worst of the epidemic is over.