There was a Navy Biplane with retractable gear, not the same thing I know..

Given the genesis of the Hurricane through the Hawker design office the addition of an additional wing was merely putting back the aeroplane back to its original roots where the design staff thought it should be.:}

I remember having a Battler Britain album where the main story involved the intrepid hero flying to France in a 'Slip Wing Hurri' to pick up a top secret captured enemy Acoustic Torpedo.
He met up with the French Resistance, loaded up the Torpedo (making a naff joke about hoping that it didn't come off in a 'hurri').
After taking off he got bounced by ME109's he jettisoned the spare wing at the right time and the chasing ME109 flew right into it, pilot shouting 'Aiiieeeeeeee'.

True story obviously.

As Daz1985 points out, the main reason for the biplane concept is to reduce wing loading (lift per unit area); this is no longer a problem for modern fighter aircraft, hence the move away from biplanes.

However, on large passenger/transport/tanker aircraft, wing loading remains a problem due to the absolute limit on wing span (about 78m to operate at a standard major airfield). The joined wing idea that Turin illustrates on the future tanker concept has many advantages: two high-aspect ratio wings are more efficient than a single low-aspect ratio one, the joined wing-tip is the ultimate in winglet design, and the two wings spread the load between two wing-root sections. The idea was considered in the A380 concept design stage but was rejected (at least partly) because potential passengers didn't like the unsusal look.

N Joe

Biplane Jets
 

I was going to respond, but most salient points have already been covered.

Most people think of biplanes with struts to connect the wings. But there are so many other solutions which may be considered as biplanes - including wings attached to fuselage at different heights and at different lengths on the fuselage. oh! Already done. Mind you do take a look at NASA and Boeings research on box wings, joined wings, bidiamond wings, fluid wings and the such like. Burt Rutan also has done some useful work as well.

It may therefore be thought that biplane jets already exist.

For further interest of another interesting technology you should look at waverider technology. Oh dear. That does bring back memories. Got involved with this technology in the past.

Hval.

(Edited for purposes of clarification)