Hi Arnold,
I am so sorry for the late response. Yes, you will be able to create 2 different objects with the same information. There is a unique ID, such as ObjectSID, for each ojbect when it is created. When you try to sync it to O365, it sees the syncing object as another object. Therefore, it create another one because the the ObjectSID is different.
I understand that you have over 100 accounts and needs to migrate to the mailbox to cloud, and it is troublesome since you cannot link the 2 accounts together. One approach is we can export mailbox from the old O365 account to the new DirSync account's mailbox. To do that, you will need to do it on Outlook by click Open> Import> "Export to a File"> Outlook Data File (.pst)
Another approach is to connect the account together, help.outlook.com/.../ms.exch.ecp.connectedaccounts.aspx
If there is anything unclear, please feel free to let me know.
Amy