Whatsapp last seen versus blue double check for sent messages I have experienced a weird situation with Whatsapp, and I don't know what it means. 
I sent a message and it got the blue double checks at 18:00 hrs, but the person to whom I sent the message was last seen at 16:00 hrs.

How could my message got  blue double checks if the person hasn't opened Whatsapp?
There could be Two (or more)  possible reason for this:
 All the word &quot;read&quot; is in past tense

Read while Offline
Your friend was connected to net, but whatsapp was closed (not in foreground) and recieved your msg.
Then he got disconnected with internet, and while this, read your message.
Thus the Whatsapp application registered that the user has read the msg.
Then your friend got connected to internet, it is now when Whatsapp fires the &quot;msg read by user&quot; request to server.
But as the Whatsapp is closed, the last seen does'nt change.

Whatsapp Plus
The second possibility is that your friend is using a modded Whatsapp version called Whatsapp Plus (discontinued for now).
It has the ability and option to hide last seen of the user, even when the user is connected to internet and Whatsapp is running in foreground.
Thus, the user can chat with you in real time without changing his last seen!
Now the thing is that, Whatsapp Plus also has an option to Hide Blue Ticks and Hide second tick.
Thus, we can conclude that, it is possible, that your friend has Hide last seen enabled, and rest all disabled.
Leading to the situation in Question.