How can I remove a duplicated entry for a child ?
Duplicated as in the same child is there with two different IDs and you are sure this is the same child and not two children with the same name? If so, you just merge them.
Or with the same ID but showing twice? Then you may be misinterpreting what you see. If this child is deceased you probably need to post the IDs so people can see the situation and give a real answer.
mod note - moved to Family Tree https://community.familysearch.org/en/categories/family-tree