Hello Dave,
Please correct me if I'm wrong but if I understood you correctly you want to display a message when you submit the form stating the Field in question is duplicated and prevent a record create / update from happening, correct?
I believe you have two distinct options here:
Server Side:
Since these records will be kept in a CDS / Dynamics environment you can create an Alternate Key which will throw an error based on the Field of your choice if it detects a duplicate. The CDS connector while making the submission to the Datasource will then show a similar error to this one:
Documentation:
https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/define-alternate-keys-portal
Note:
Please note that if you already have duplicates in the system, make sure you resolve them before creating the Alternate Key otherwise it won't work as the System Job that will create will fail.
Client Side:
You can also perform a check before submitting the form by performing a request against the Data Source and looking for any records with that same Field Value. You can use the Lookup() function to do so.
Example:
LookUp(Accounts, 'Account Name' = DataCardValue.Text) --> This will return the first record that meets the condition. Which means that if a record is returned a duplicate exists and you should Notify the user and abort or prevent the form submission.
Hope this helps 🙂
Filipe Relvas.
