A Dandy in Aspic (1968)

Double-Agent Alexander Eberlin (Laurence Harvey) is assigned by the British to hunt out a Russian spy, known to them as Krasnevin. Only Eberlin knows that Krasnevin is none other than himself. Accompanying him on his mission is a ruthless partner, who gradually discovers his secret as Eberlin tries to maneuver himself out of a desperate situation.