The problem is that your old field and new field has the same name. The old field stored a post ID value of the attachment file, let’s say that ID is 1000.
For a repeater field ACF stores several values, the values stored for the repeater itself is then number of rows in the repeater. When ACF get’s this value it creates that number of rows.
1) You need to use a different name for the repeater than you used for the original field
2) You cannot transfer existing values from your original field into rows of the repeater, at least not without a lot of work.
