This isn’t something that can be done using PHP. When a repeater field is generated there is a ‘clone’ version of the row that is hidden. When you add a row ACF copies that hidden row to make the new row.
There is nothing in ACF that will let you conditionally change the label of a field, or any other aspect of a field, based on another field. I’m not the developer, but I honestly don’t see features like this making it into ACF.
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