Agree with GoldDiggin, make a 3 gallon imperial (higher gravity) version of the clone you were going to make, but use the appropriate amount of hops, and instead of adding to secondary, I would ferment the second version until its done and then blend the two using taste tests to find the right amount to add.
Another option would be to add vodka or bourbon to the low gravity beer to bring the alcohol up.
I make a large amount of hard cider and sometimes blend cider with beer.
Somewhat radical would be to draw off 1.5 gallons of the existing beer, heat it high enough to sterilize the DME or LME, chill it and add back to fermentor.
You don't have to boil it, 160-175f should be enough.
Another option is to put the beer in 1 qt to 1 gallon square containers (when its done fermenting) freeze it and then pull the block of ice out, leaving a higher alcohol "beer". Would be an interesting experiment, but you'd have to take steps to avoid contamination, like pasteurizing before bottling.
Someone said just dump it, but I'd tinker with it, maybe split it up and try 3 different things, but you need some extra jugs or carboys to do that.
Good Luck, Cheers!!