In Germany, it's common to have cities in different parts of Germany with the same name. In such cases, one usually puts a nearby geographic feature (usually a river) or region next to the name of the city.
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.
It basically comes down to data availability, and some historian digging through the available archives.
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).
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.