The In/N/Out burger is not unique. There are other places that make basically the same burger but do something more that makes for a good stop. I have listed two places below that make a burger that looks, smells, feels and taste like an In/N/Out burger but wonder whether others exist?
Art's ,The Art of Burgers (Maybe even better meat than In/N/Out, Wed and Sun the burgers are only 99 cents. Try a 3X3. Add chili and it goes well beyond In/N/Out. The menu includes chili dogs, pastrami, breakfast burritos, and a pretty good fish sandwich too)
11629 E. Valley Blvd. (East side of the 10 Fry. on the North Side of Valley Blvd. Look for a building that looks like a original In/N/Out from about fifty years ago. I hear that it was one of the original two-lane drive-thu In/N/Outs)
El Monte CA 91732
Baker's Burgers (What makes this better then a stop at In/N/Out is the real ice cream milk shake – OMG!, I had two of those chocolate shakes with my burger last week. Shame on me)
2555 S Archibald Ave (North side of rchibald Ave at the 10 Fry -- Ontario Airport)
Ontario, CA 91761