Re-purposing story lines
On the topic of "self-plagiarism," note that some of the Rigg era Avengers episodes borrow from earlier (British) Avengers episodes. The Rigg era piece "The 50,000 Breakfast" is very close to "Death of a Great Dane."
Of note is "The Superlative Seven" which borrowed heavily
from a Macnee/Blackman episode "Dressed to Kill":
"Dressed to Kill" was one of the last great Cathy Gale episodes, scripted in exciting fashion by Brian Clemens, soon to perform double duty as writer-producer for the next season with Diana Rigg. Steed is among seven people gathered together on a train for a New Year's Eve party, all dressed in different costumes, who become stranded at an abandoned railway station, targeted for their various plots of land near an early-warning radar facility. Many later episodes would borrow elements from this story, particularly "The Superlative Seven," whose twists and turns actually surpass those found here. Among the superlative cast, Richard Leech had previously appeared in "Traitor in Zebra," and Frank Maher had done "November Five," while John Junkin (soon to appear opposite The Beatles in 1964's "A Hard Day's Night") went on to do "Never, Never Say Die." Even with Leonard Rossiter and Alexander Davion also on hand, it's Anneke Wills ("The 50,000 Breakfast") who steals every scene as the Pussy Cat, complete with adorable ears and attractive tail, understandably taking an instant liking to Steed's cowboy garb ("careful, you'll make me purr!"), leading to a 'High Noon' showdown on the platform, with a little bit of help from a disguised Mrs. Gale, wielding a six shooter of her own. Lovely blonde Anneke Wills would soon marry future AVENGERS villain Michael Gough, lasting from 1965-1979.
Each of the guests in each story were invited by a different host (Steed by Tony Linkletter in "Dressed to Kill")
In "Dressed to Kill," Napoleon was the culprit and one of the last lines in the episode was:
It's a long way from Waterloo, Station that is.
Honor Blackman to Patrick MacNee: Don't push your luck Steed, we only just got through with this one.
The party in "Dressed to Kill" had a New Year's theme and aired 28 Dec. 1963 .
Wikipedia also notes: "The Joker" was to a large extent a re-write color episode of the earlier Cathy Gale b/w era story, "Don't Look Behind You," as were a few other later episodes re-writes in color of b/w era tales.)
As a bit of trivia, Anneke Wills is in both Dressed to Kill (with Blackman) and "The 50,000 Pound Breakfast (with Rigg).