“Opportunity knocks but once” is a proverb that dates, in various forms, to the 1500s. “Opportunity knocks only once, but temptation leans on the doorbell” is a jocular extension of the proverb that has been cited in print since at least 1960. The author is unknown.
   
“Opportunity may knock only once, but temptation leans on the doorbell” has been printed on many posters.
 
       
14 March 1960, The Robesonian (Lumberton, NC), pg. 8, col. 2:
Opportunity knocks only once, but temptation leans on the doorbell.
 
8 June 1961, The Star (Park Forest, IL), pg. 5, col. 5:
Opportunity knocks only once, but temptation leans on the doorbell.
 
11 February 1965, The News (Van Nuys, CA), pg. 9-B, col. 5:
Opportunity knocks only once, but temptation leans on the doorbell.
 
29 April 1968, Albert Lea (MN) Tribune, “Music Concert Presented By Ellendale Musicians” by pg. 7, col. 6:
That is about all the news from Ellendale that I have for this week but I’d like to pass along this little thought—‘‘Opportunity knocks only once, but temptation leans on the doorbell!”
   
Google Books
The School Library:
A Force for Educational Excellence
By Ruth Ann Davies
New York, NY: R. R. Bowker Co.
1969
Pg. 131:
“Opportunity knocks but once, but temptation leans on the doorbell.” Western Livestock Journal
 
Google Books
Wickedly Yours
By Brenda Hiatt
New York, NY: Avon Books
2003
Pg. 517:
The plaque over my friend’s computer says it all: Opportunity only knocks once; temptation leans on the doorbell.
     
Google Books
Phillips’ Treasury of Humorous Quotations
By Bob Phillips
Wheaton, IL: Tyndale House Publishers
2004
Pg. 252:
Opportunity knocks but once, but temptation leans on the doorbell.
 
Google Books
Words from the Wise
By Rosemarie Jarski
New York, NY: Skyhorse Publishing
2007
Pg. ?:
Opportunity may knock only once, but temptation leans on the doorbell.
Anon
 
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Opportunity may knock only once, but temptation leans on the doorbell.
5:00 PM - 12 Feb 2015