How do I migrate a large iCloud mailbox to Gmail without duplicate emails?

Migrating a large iCloud mailbox to Gmail can become complicated if the same emails are transferred more than once. To avoid unnecessary duplicates, you can use a migration tool that provides built-in duplicate detection. SysInfo iCloud Backup Tool supports direct iCloud-to-Gmail migration and can identify duplicate emails using details such as To, From, Bcc, Subject, and Body. You can sign in to your iCloud account, select the required folders, and choose Gmail as the destination. Before starting the migration, enable the duplicate-handling option and apply a date filter if you only want to transfer emails from a specific period. The tool also provides an option to preserve the original folder hierarchy, helping keep the mailbox organized after migration. For a very large mailbox, transferring emails in smaller date-based batches can also make it easier to check the results and ensure that the important messages have been moved correctly.