Why the co-author's information is being overwritten?

Here's what has happened:

If a co-author already exists in the system (probably because another submitter submitted an abstract and added this co-author), the system does not allow the submitter to make any changes to the co-author's profile.

So, if "Submitter" has added "co-authors" who already have a profile in the system from a different abstract submitted earlier, the "Submitter" would be able to add them to her abstract but would not be able to update their profile.

The reason system does not allow submitter to add information about a co-author who exists in the system is to prevent any submitter from being able to edit their profile information they have added on their own. 

An admin can update the co-author's information from under Contacts Management in the backend, however.