“Why does the bar association prohibit lawyers and clients from having sex?” (lawyer joke)

One popular lawyer joke involves lawyers screwing their clients:
 
Q. Why does the bar association prohibit lawyers from having sex with clients?
A. They don’t want clients to be screwed twice and only to pay once.
 
A differently worded punchline is, “To prevent clients from being billed twice for essentially the same service.” The joke has been cited in print since at least 1993.
 
     
Los Angeles (CA) Times
Call to End Lawyer-Bashing Inspires Even More Jokes : Backlash: Radio listeners condemn the profession. State Bar chief’s campaign even annoys some attorneys.
July 08, 1993 | MAURA DOLAN and HECTOR TOBAR | TIMES STAFF WRITERS
(...)
Taking aim, he added: “This is the same State Bar that came out last year against sex between attorneys and their clients. They didn’t want the clients to be double-billed for essentially the same service.”
 
Goole Groups: alt.tasteless.jokes
Warning long one…Lawyer jokes
Laura Schooler
7/8/96
(...)
Q. why doesn’t the ABA condone sex between lawyers and clients?
A. to avoid having clients billed twice for what is essentially the same service.
 
Google Groups: alt.religion.mormon
MORMON FALSEHOODS:
TTaylor224
7/3/99
(...)
Q. Why does the bar association prohibit lawyers from having sex with clients?
A. They don’t want clients to be screwed twice and only to pay once.
 
Google Groups: rec.music.opera
Attorneys…
Little Jimmy Olsen
12/21/02
(...)
2. Q. Why does the Bar Association prohibit sex between lawyers and their clients?
A. To prevent clients from being billed twice for essentially the same service.
 
Google Groups: alt.alien.visitors
A different type of joke for the NG’s
Sir Gilligan Horry
5/2/04
Q: Why does the bar association prohibit lawyers and clients from having sex?
A: To prevent clients from being billed twice for essentially the same service.
 
Google Books
Man Walks into a Bar:
Over 6,000 of the Most Hilarious Jokes, Funniest Insults, and Gut-Busting One-Liners
By Stephen Arnott and Mike Haskins
Berkeley, CA: Ulysses Press
2007
Pg. 250:
Why does the Bar Association prohibit lawyers and clients from having sex?
To prevent clients from being billed twice for the same service.
 
Google Books
Hilarious Lawyer Jokes:
An Illustrated Caseload of Jurisprudential Jests
By Steven D. Price
New York, NY: Skyhorse Publishing
2015
Pg. ?:
Q: Why does the bar association prohibit lawyers from having sex with their clients?
A: To prevent clients from being billed twice for the same service.