A bit difficult to describe, but it’s happened multiple times.
I have a field group with nine fields. The last one is a repeater, containing four fields. After making changes to the repeater fields (eg changing their type of fields 2 and 3 from “text” to “number”) and saving, two new rows appear in the main field group. The rows correspond to the repeater rows just changed (eg 2 and 3 in the example above).
It’s not a huge problem since I can simply delete the incorrectly added rows.
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Thanks for the bug report,
Elliot here – ACF dev.
Just wanted to let you know we found and fixed this issue in the past week.
If possible, can you please try our latest build (5.7.0-RC1). We have made a lot of improvements over the past weeks and would like to hear if this has solved your issue.