You can migrate all the mailbox items to Office 365 from On premise Exchange server without any complexity . This is possible using third party tool i.e EdbMails Exchange Migration tool. This tool avoids your worries of duplicate mailbox items during migration as it provides the incremental feature for the consecutive migration on the same system. You can migrate any size of mailboxes from any Exchange server versions.

To know more Upgrade Exchange 2010 to 2016 | Exchange Migration | Exchange to office 365 Migration