Are you trying to prevent duplicates in a certain field in SP? If so then the best way would be to mark the corresponding field as "unique". In this case you could use a regular form submit functionality and it will show user a warning on an attempt to store duplicate values.
If the duplicates criteria is more complex then one way would be to use the LookUp function to see if the records exists and then call SubmitForm or Patch. For example, your Submit Button.OnSelect could be set to something like:
Condition_To_Check_For_Dupplicates would be comparing the value from thr data card from your form with the value from the SP field. For example, if the your data card value is DataCardValue10 and the SP field name is MyTurno then the condition would be DataCard.Text=MyTurno
Could you please share a bit more about your PowerApps app?
I agree with @dinusc's thought almost, I assume that you have a unique column (which could identify one record uniquely) in your SharePoint list, you could also take a try with the following workaround:
Within EditScreen, the OnSelect property of the Submit button set to following formula:
Note: The '20180521_case11' represents the SharePoint list data source in my PowerApps app. The ProductName is a column in my SharePoint list, which has enabled "Enforce Unqiue Values" option. The DataCardValue10.Text is used to reference the value of the ProductName property within EditForm control.
Community Support Team _ Kris Dai If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.