If a ticket is opened by another user when you open it, the system will let you know this by posting a message. However you can still change it. If you want to lock the Ticket so another user can view the content, but not make any changes when opening the same, click the lock icon in the Ticket top right corner. This works both in SSP and Agent portal (View and Edit mode).
Exception! Tasks can still be added by other users when the lock is on.