Star Trek: The Original Series
The Enemy Within
S1 E5:
A magnetic ore brought back from Alfa 177 causes a beam break-up in the transporter, splitting Captain Kirk into two different beings: one "good" and one "evil." The malfunction must be repaired before Sulu's stranded landing party freezes to death -- and Kirk's separated "halves" perish.
Starring
Grace Lee Whitney, Eddie Paskey, Garland Thompson