I thought the very strangest casting choices for guest stars were the ones where a recognizable guest star repeated in a very different role....
I guess things were different in the 60's.
Not only was that par for the course in the '60s and '70s, it still happens from time to time today. On Law and Order
, Jeremy Sisto played (IIRC) a defense attorney in the 17th-season finale and then got cast as one of the regular cops in the 18th-season premiere. And heck, the next star of Doctor Who
is Peter Capaldi, who's played two previous roles in the franchise -- and former companions Freema Agyeman and Karen Gillan had also played other guest characters not long before. (As did several other regular or recurring players in the original series, but I'm focusing on modern instances.)
But yeah, it did happen all the time in '60s and '70s TV, when nobody expected old episodes to be remembered for long and continuity wasn't an issue. Generally they tried to avoid having an actor play two different characters in the same season, unless it was a minor background player, but there were a lot of shows where you'd see the same actors come back roughly once a season in different roles each time.