Emma would only record the most recent update to any particular contact. This can be seen by clicking into an audience member's contact record and viewing the 'last modified' time stamp towards the top of the record.
If a contact’s status is changed, email address is updated, confirms opt-in or if the contact record is deleted entirely, the timestamp will be updated.
The timestamp is not be updated for contact field changes nor when group membership changes.
Last modified timestamp WILL update upon an “add and update” import when a contact field is changed. This is true for all contacts including error and opt-outs.
Lastly, you can see historical imports by going to audience > imports. This will also show you where a particular sheet was imported to at that time.