If new data is imported, please note the information in the Import/Export module and in the file. 
Neue Daten dürfen keinen Wert im Feld „ID“ haben. 

If a data record is changed, the ID must be entered in the ID field. 

You will find the IDs when you perform an export in the respective row in your Excel file. 

If you overwrite data, we can not undo.
It is always advisable to perform an export before importing new data.