Enterprise Vault should only have one entry for each user in the ExchangeMailBoxEntry table. Therefore if you have two entries that correspond to two different Exchange Servers (ExchserverIdentity column, which relates to ExchangeServerEntry table) then the one that references the incorrect Exchange Server requires removing.
For two entries, there was indeed a difference for the ExchangeServerIdentity. I removed the wrong entries. For the other two, they were the same. However, some columns had a NULL value. I decided to remove the 2 entries having the value NULL instead of a number, and that has worked out ok.
Thanks for support