When you copy a person from your private list, you will create duplicate records, and that is OK.
The people you add may already exist in the private tree of another user. Since you cannot see that private tree, and since the Family Tree find feature does not include living people in search results, you will not be able to tell.
How family group trees could reduce duplication in the long run
Over time, we hope that FamilySearch users add fewer living people into their private trees and more into family group trees. When the family group tree feature is released, we recommend that you add just enough of your living ancestors into your private tree that you can connect to your deceased ancestors.
If your private tree has many other living family members, we recommend moving that information into a family group tree. Over time, this will reduce the number of duplicates in the tree. It will also allow a larger group of users to update a person's tree profile.
Can I merge duplicates in my family group tree?
Yes. You can merge duplicates if they are in the same Family Group Tree. Use the Merge by ID feature.
Can I merge duplicates from different trees?
No. The duplicates must be in the same family group tree or private tree.
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Merge duplicate records of living and confidential people in Family Tree