What is the reason for a repeating entry when updating through the parent in a Has Many relationship?

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The reason for a repeating entry when updating through the parent in a Has Many relationship is that the child table has multiple matching records for the parent record. This can happen when there are multiple records in the child table that have a foreign key that points to the same primary key value in the parent table. When updating through the parent, the update query will affect all the matching child records, resulting in duplicate updates.