This is a famous phrase from the 1980 presidential campaign -- it was a way for Reagan to "disarm" his opponent during debates and most effective against incumbent Jimmy Carter.
But, of course, this is not a politics DBD. Political discussion is actually banned from he... oh wait, what's that? They lifted that ban years ago? Okay, never mind then.
Anyway, what are the things that people repeatedly say that make you go "There you go again" in your head? When stanfurd kids pretend they followed football before Andrew Luck? When UCLA alumni talk about how their academics are just as good as Cal's? The boss talking about the importance of leveraging synergy outside-the-box corporate diversity? The entire wine snobbery circlejerk from yesterday's DBD?
For me, it's the "that's not authentic" line, used when referring to food. It's one thing to share knowledge about how diverse and different food is in Italy vs what we know as "Italian food" here. But I'm pretty sure most of us know that Taco Bell and Panda Express are not even trying to be authentic. They're fast food places that people enjoy.
There you go again.