Not only was that par for the course in the '60s and '70s, it still happens from time to time today.
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.
I didn't care for Capaldi's character in the Torchwood episode at all and I thought he was just average in Fires of Pompeii. Those are the only 2 times I saw him so I wasn't thrilled when he was chosen as the next Doctor. I think a cool twist would be for them to remake Fires of Pompeii where it's revealed that Capaldi's role is really that of the 12th Doctor and for some reason he had to "cross his own timeline" and interact with his former self.
Freema Agyeman's previous character was explained as being her cousin. Yeah, a bit of a cop out but at least they addressed it instead of completely ignoring it.
Karen Gillan was hidden in makeup and I had no clue that she had previously played in an episode. Even going back and re-watching it, she wasn't instantly recognizable.
Back to Trek - as others have mentioned, Mark Lenard's role as the Romulan Commander and also Spock's father really had me confused. I initially wondered what the heck Spock's father was doing on the Romulan ship and the fact that Spock and the rest of the crew showed surprise when seeing him only reinforced my misunderstanding. Why choose the same actor, put him in the same makeup (Romulans looked just like Vulcans) and have both characters in a strong role was a strange choice in my opinion.