On Magnum P.I. it seemed they were always using the “twin” ploy or a twist on it. Higgins had two or three brothers who were virtually identical but markedly different in personality. There was that ensign who was either a twin or pretended to be a twin who reappeared in a couple of episodes.
And don’t forget Morticia Addams on the Addams Family – OK, OK they were identical cousins. But the cool thing about them was that the evil twin was in fact the good twin or was it the other way around?
In the film Smokey and the Bandit 3 (OK, there were only seven of us that saw that one) Jackie Gleason played triplets - oh, I forgot, all pretty bad, so no good.
On the Flintstones, the Great Gazoo creates doubles of Fred & Barney. Only problem is, one always says “Yes-yes-yes!”; the other, “No-no-no!” And nothing else.
They also had his buddy, Mac, get blown up in the Ferrari, then a Mac lookalike was featured in a later episode.
On an episode of The Incredible Hulk, Bill Bixby played David Banner as well as some organized crime boss who wanted to use Banner as a decoy to get him killed. I think someone tried to dump concrete on Banner for a Hoffa kind of demise and he went all Hulky.
In an episode ot the original The Bionic Woman, a Soviet spy is surgically altered to resemble Jaime Summers, and uses some kind of drug to simulate her super-strength.
Also, the Fembots had android replicas of several O.S.I. personnel.
The title character of The Man from Atlantis had an “evil” twin. Who had travelled through a time warp and became a gunfighter in the wild west. (One of the wierder episodes in a show that was meant to be wierd.)
None of them were twins or clones or anything, but in Edgar Rice Burrough’s novels, there seemed to be quite a lot of guys running around Africa who were dead ringers for Tarzan.