Double struck 2012 £2 coin

My brother found a very cool double struck 2 pound coin with the tail side imprinted over the Queen's face anyone have an idea of how rare this might be?

 Guessing this is the coin 

N#1324 

 with all those wavy lines. No idea of value - 

the error is called a brockage. 

https://en.wikipedia.org/wiki/Brockage 

Wow thanks pretty cool, I've told him that the best thing to do is look after it and keep hold of it long term as it may be when rth more years down the line