Is there a way to automatically show a pop up message when there are lines that contain the same item in a transaction without using scripting?

No

Makes sense. If I want this to be real time, then I need a client script, right? Do you have an idea on how the general logic would look like? When a user clicks the Add button in the transaction line, they get should get a pop up message. For CSV import, I want to fail the import in such cases. So I guess this would be user event script before submit. But do you have an idea as to the general logic? Need to loop through the lines, etc.? Thanks.

UE before submit yes you could loop the lines, check if the item exists in an array.

couldn't you use the show message workflow action for the item duplicate? only csv should be scripted (if that's even possbile)

@Kevin Zwemmer how would the system know if there's a duplicate line?