“Toughest job I ever had was as a door to door salesman, selling doors” (joke)

There is a joke about a door-to-door salesman who had an especially hard job—he sold doors,  “Dad has the hardest job in the world. He’s a door-to-door salesman selling doors” was printed in 1956 newspapers. “It’s hard being a door-to-door salesman. Especially if you’re selling doors. Because right away you know its giving them something they already have” was printed in a 2006 book.
   
“My worst job was being a door to door salesman, selling doors. *knock knock* ‘Hi Madam, are you interested in our, oh, shit, you have one’” was posted on Twitter on January 26, 2009.
 
         
16 November 1956, Salisbury (MD) Times, “Small Fry Diary” by Nonnee Coan, pg. 31, col. 2:
FRIDAY: “Dad has the hardest job in the world. He’s a door-to-door salesman selling doors.”
 
Google Books
The CollegeHumor Guide To College:
Selling Kidneys for Beer Money, Sleeping with Your Professors, Majoring in Communications, and Other Really Good Ideas
By Writers of CollegeHumor.com
New York, NY: Dutton
2006
Pg. ?:
Its hard being a door-to-door salesman. Especially if youre selling doors. Because right away you know its giving them something they already have.
   
20 October 2008, The Sun (London, UK), “Have a laugh!” with Bill Bailey, pg. 2:
MY first ever job was as a door-to-door salesman - selling doors. That’s a really tough job.
 
Twitter
Tim Hanson‏
@TimElmo
My worst job was being a door to door salesman, selling doors. *knock knock* “Hi Madam, are you interested in our, oh, shit, you have one.”
6:41 PM - 29 Jan 2009
 
Twitter
Matt Kempster‏
@kempster_old
“My first job was a door-to-door salesman selling doors” - channel 4
5:39 PM - 20 Mar 2009
 
Twitter
☜ Mrs. Solo Dolo‏
@GenieHendrix
“Its hard to be a door to door salesman. Especially if you’re selling doors. Because right away you know…” http://tumblr.com/xog2xx4do
7:13 AM - 2 Sep 2009
 
Twitter
jenny houghton‏
@JenniiMarie
It must be hard to be a door to door salesman selling doors, they walk up the garden path and think ‘Damn.. they already have one’!
9:15 AM - 7 Nov 2009
 
Reddit—3amjokes
Toughest job I ever had was as a door to door salesman, selling doors… self.3amjokes
Submitted September 19, 2017 by madazzahatter
Every time I knocked, I thought, “Oh man, they’ve already got one.”