Exchange Online (Office 365) not copying duplicate messages during migration? Which migration method to choose?

Question

Mail server

Good day everyone!
We are migrating from a Linux ground-based mail server to the cloud-based Exchange Online, which is part of Office 365.
We are currently at the stage of testing copying the message base from the ground to the cloud.
Exchange Online allows you to copy messages automatically over IMAP.
Tests have shown that the number of messages in the original boxes on the ground is somewhat different from the number of messages in the ground boxes on the Linux server.
Through some torment, we found out that ExchangeOnline does not copy duplicate messages.
Of course, there were questions, the answers to which I would like to receive here:
1. Can I have Exchange Online copy all messages, including duplicates during the migration process? We are talking only about migration via IMAP using Exchange Online.
2. What criteria does Exchange Online use to determine duplicates? It is necessary in order to be able to compare a land-based mailbox and a cloud one in terms of the number of letters. And since there will be duplicates on earth, they need to be excluded according to some algorithm. This algorithm must be the same as for Exchange Online.
3. I assume that the first question I will receive a negative answer, so I'm asking this question. What other migration methods would make sense to consider? Conditions: about 1000 boxes, dozens of folders in each box (so copying by hand is not an option), maximum automation of the process. Exporting messages from an old mailbox to a PST file is not good. Copying hands on IMAP of each folder - is not good. If there is a smart transfusion via IMAP - please give me a tip, perhaps this will be the way out.
Thanks!!