The reason for the duplicate response is that the friendlyNames parameter for that interface defaults to the item label name. To prevent that, you could set that parameter as “Wert” but I suspect you will run with the same issue mentioned in the post below related to the German language.
Unfortunately, only unit of measure part of the supported list can be used. This is a limitation on the Alexa side.
Number:Dimensionless Netatmo_Workroom_CO2_Alexa "Arbeitszimmer CO2" { channel = "netatmo:NAModule4:home:03000009xxxx:Co2", alexa="RangeController.rangeValue" [friendlyNames="Wert", supportedRange="300:4000:1", nonControllable=true] }