Mike and John dined at the Taco Palace restaurant.  They each ordered the Taco Grande Plate as their main meal, but Mike also ordered a side salad for $2, a plate of cheesy fries for $4, and a diet cola for $2.  As a result, Mike's lunch bill was twice as large as John's bill.  What was the combined total cost, in dollars, of Mike and John's lunch?
Please reason step by step, and put your final answer within \boxed{}.
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