Hi Hannes,
> The client is entering the data manually, and as a precaution there are two fields set up so that the number must be entered twice, reducing the potential for entering a wrong number.
> ... are you looking for a validation of the data and provide feedback if the user entered different information for both fields?
I can confirm that Survey123 can indeed be enabled to do what you're asking for. You can enable this type of business logic and validation with smart forms, which is a key component of Survey123.
To learn more about Survey123 for ArcGIS:
Hope this helps,