This seems to be a general file lock issue as ArcGIS expects exclusive file locks on shapfiles and file geodatabases. Check any %computer-name%.lock inside the folder to find which computer is using this file.
However, if anyone like me came here to looks for a solution for cases in which files being stored on NAS or other network storage, you might want to check your OS ver. (win7 and win2008) and disable SMB2 Caching as @lior has suggested.
ESRI Doc has describe this caching system would refresh the remote folder only every 10 secs while ArcGIS would check the success of file locking by looking after the .lock file almost instantly.
To disable the caching on win7 and win2008, update your registry by creating a new dword entry and reboot.
Windows Registry Editor Version 5.00
Microsoft's Doc about this