"You can’t con a con man/artist” is sometimes used in political arguments. The cynical premise is that all politicians are con men (or women), so that one can’t easily con them. “You can’t con a con man” has been cited in print since at least the early 1960s.

A similar phrase is “You can’t BS a BSer"("You can’t bullshit a bullshitter").


Google News Archive
10 February 1963, St. Petersburg (FL) Times, “Casey Stengel Cons Con Man” by Bill Veeck, pg. 3C, col. 1:
So I thought, “What kind of a fool does Stengel think I am, trying to con me by throwing in the scout’s name like that? Doesn’t that old fraud know you can’t con a con man?”

Google Books
The Progressive
v. 28 - 1964
Pg. 33:
“You can’t ‘con’ a ‘con man’,” and addicts are expert “con men.”

Google Books
The Namath effect
By Marty Ralbovsky
Englewood Cliffs, NJ: Prentice-Hall
1976
Pg. 196:
“You can’t con a con man, you know, and I’m just happy I don’t have to be a con man anymore.”

Google Books
November 1983, Yoga Journal, pg. 22, col. 2:
“The old saying is true, ‘You can’t con a con.’”