We've found the following important issues in rekordbox 4.2.0 that was released on 1 July 2016:

• Sync manager sometimes functions incorrectly.
• Tracks may be duplicated or deleted when using playlists.
• Load error message may appear and software may “crash” when using CDJ/XDJ via PRO DJ LINK.

We have fixed the above issues in rekordbox 4.2.0b.

Please download and install the rekordbox 4.2.0b from here.