I'm assuming that the record is locked so that only one user - presumably the first - can edit, whilst others are locked out until the record has been saved. But I don't know, and a client has just raised the issue, so I'd like to provide a definitive answer.
Tested on CiviCRM 4.7: two users can edit the same contact record, however, at the moment of saving the contact record, the second user to save the contact record will get a warning that the contact record has been modified by someone else. The user will then have an option to save anyway or reload the page.
It also works with inline edit:
Here is an older thread with some changes that went into 4.3 (not sure if there have been further changes since then):
- https://issues.civicrm.org/jira/browse/CRM-10553 ("Implement contact-locking")
Note: Initial discussions were held via email with followups on IRC on Monday (16-Jul-12) and Wednesday (18-Jul-12). An extra discussion document was included on Wed ( http://pastebin.com/BieCSzXa ). A blog post explored the issue further. Another IRC conversation (13-Aug-12) settled on optimistic locking.
Some key design decisions:
- Locks are optimistic
- Locks apply coarsely to all core contact tables -- civicrm_contact, civicrm_address, civicrm_phone, civicrm_email, civicrm_website, civicrm_im, and single-value custom-data
- The key used to check revisions will be a "modified" timestamp on civicrm_contact
- Locks are only checked for the "Edit Contact" and "View Contact - Inline Edit" screens. Other screens will not read the "modified" timestamp or display warnings/errors about it; however, other screens will update the "modified" timestamp.