Under the parent's Children, press What are you trying to fix?, then Something else…, and choose the option saying a child is in the tree twice. One daughter, two cards? Let's make it one! The parent's other children show first, and a likely match is marked part green to help you pick. Then the merge page takes you through it, just as for any duplicate. For example, from your uncle's card you can merge Amy and Amy Louise into one daughter. It works from the child's own card too.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
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