As Eater Houston notes, the sandwiches are essentially identical — two patties and two slices of cheese between two buns — except that Underbelly goes the extra mile for freshness by butchering the meat in-restaurant and growing all the vegetables themselves.
Whether or not Shepherd was truly unaware of the cult burger chain's flagship sandwich didn't matter much to In-N-Out, which had its lawyers fire off a cease and desist letter telling Underbelly to quit using their trademarked burger name at once.
And so they did:
In-N-Out Burger has, of course, heard of our Double Double Burger. Henceforth known as the "Cease and Desist Burger." #AskForItByName— Underbelly (@UnderbellyHOU) June 18, 2013
With all due respect to Mr. Stephen B. Kaplitt, that's how you respond to a cease and desist letter.