After thorough research, I am not able to find an answer to my simple question, unfortunately.
I have a SharePoint list with e.g. 5'000 entries:
I have created a user form for end-users to be able to add new entries to this list. It parses a text copied by a user from Excel and creates a collection that later gets Patched to this SharePoint list (it creates new items in this SharePoint list):
However, I am struggling to understand what would be the best way to do a check if an entry with this CustomerCode already exists in the SharePoint list to avoid any duplicates to be uploaded by a user? In this example, CustomerCode "X001" marked in red color should not be added to our SharePoint list because it already exists. Any ideas on how to accomplish this?