While doubled die obverses are common in the shield nickel series, doubled die reverses are decidedly uncommon. This 1870 is of additional interest because the doubling is the result of hubbings from two different reverse designs - this is a reverse of '70 (reverse IIc) impressed over a reverse of '67 (reverse IIa).
This variety is known to come paired with two different obverse dies.
Runner up is another 1870 doubled die reverse. This is another instance of doubling occurring because two different design hubs were used, but in this case it is exactly the opposite of F-14 above - this is reverse of '67 impressed over reverse of '70.
Even more interesting is that most reference books list an 1870 with reverse of '67 as "thought to exist". We now know it does exist, but only as a doubled die.
The F-14 gets the nod here because it is a more dramatic variety.