Diana Muldaur played two different characters in the original series before become Kate Pulaski in TNG.

Robert Duncan McNeill played Nick Locarno on TNG; this character was almost identical to Tom Paris on Voyager.

Jeffrey Combs played two unrelated recurring characters on DS9 (Brunt and Weyoun), even playing both in the same episode (”The Dogs of War”), as well as playing Shran on Enterprise. Of course, Combs is a good enough actor that little disbelief was required, even in the other three roles he played on DS9, Voyager, and Enterprise respectively. (Makeup helped too.)

Not to mention Marc Alaimo, Vaughan Armstrong, Rene Auberjonois, William Campbell, Mark Lenard, Ethan Philips, Tim Russ, Armin Shimerman, and many, many others…

In the early days of M*A*S*H, Morgan did a guest starring gig as a general (Steele, I believe) who, during the course of the episode, was found to be insane. Later, following the departure of McLean Stevenson as Lt. Col. Henry Blake, Morgan returned to the 4077th, this time as its commander, the beloved Col. Sherman T. Potter.