“Knock, knock.”/“Who’s there?”/“Doctor.”/“Doctor who?”

Doctor Who (a British science fiction television series) is included in a popular knock-knock joke. The person at home asks, “Who’s there?” The answer is, “Doctor.” The person at home then replies, “Doctor who?”
 
The joke has been cited in print since at least 1976. Final lines after “Doctor who?” include “Exactly” and “That’s right” and “No, just a regular doctor.”
 
   
Wikipedia: Doctor Who
Doctor Who is a British science fiction television series broadcast by the BBC since 1963. The series depicts the adventures of a Time Lord called the Doctor, an extraterrestrial being who appears to be human. The Doctor explores the universe in a time-travelling space ship called the TARDIS. The TARDIS exterior appears as a blue British police box, which was a common sight in Britain in 1963 when the series first aired. With various companions, the Doctor combats foes, works to save civilisations, and helps people in need.
       
Newspapers.com
7 May 1976, Ealing (UK) Gazette, “Laughing Members Write Their Favourite Jokes,” pg. 19, col. 2:
Knock, knock!—Who’s there.
Doctor.—Doctor Who?
That’s right!
     
Newspapers.com
25 January 1980, Ealing (UK) Gazette, “Lots of jokes,” pg. 10, col. 3:
Knock. Knock.
Who’s there?
Doctor.
Doctor Who.
Doctor Who.
 
Twitter
Heidi
@heidihoopes
Also, Knock knock. Who’s there? Doctor! Doctor who? Doctor Who travels through time and space in the TARDIS! (Katya appreciated it)
6:35 PM · Mar 19, 2008
   
Twitter
Adrián Bertolini
@adbert
-Knock, knock! - Who’s there? - Doctor - Doctor Who? -That’s right!
11:22 AM · Dec 13, 2008
   
Twitter
Katie Seggons
@Oi_angrycat
@UniqueUsernameN GENIUS! ... “Knock knock.” “Who’s there?” “Doctor.” “Doctor Who?” “Ahh, so you’ve been expecting me.”
9:33 PM · Oct 16, 2009
   
Reddit—Doctorwho

Posted by u/apalapachia April 11, 2012
Knock knock joke I came up with..
Knock knock. Who’s there? Doctor. Doctor Who? Just the Doctor.
COMMENTS
PrincessPi
David Tennant told this joke on Nevermind the Buzzcocks.
(...)
thefluttershytree
[To a friend who doesn’t like sci-fi] Knock knock! Who’s there? Doctor. Doctor Who? I thought you said you didn’t like science fiction!
(...)
SlaveKeyboardist
Knock, knock Who’s there? The Doctor! Doctor Who? Exactly my dear boy!
 
Twitter
Wismerhill
@Gordan_Xor
-Knock, knock !
-Who’s here ?
-The doctor
-Doctor who ?
-Exactly !
😁
#DoctorWho
10:11 AM · Apr 28, 2023
 
Twitter
andy rimmer
@ianandrewrimmer
Knock, knock
‘Who’s there?’
‘Doctor.’
‘Doctor Who?’
‘Has to be ... real doctors don’t do
house calls anymore!
#WednesdayWit
3:15 AM · May 17, 2023