Update error - Failed to get lock on file

[Cerc]

Reading package lists...
E: Failed to get lock on file /var/lib/apt/lists/lock. It is being held by process 1691 (apt-get)
E: Unable to lock directory /var/lib/apt/lists/

How fix it?

[stevef]

That's a slightly unusual message.
Can you tell us the sequence of events leading to this ?
Have you tried re-running the install updates ?