Line in 3/4 bleeding into miked channel one

I am recording vocals to my original accompaniment. Mike on channel 1, computer playing accomp on Line In 3/4. Recording on two channels on Audacity. On playback I hear the accomp in the background of my vocal, even tho they are on separate channels. When I record directly with the computer (Mac) mike, the accomp is not in the background. Why is the Line in input bleeding into channel one? Tnx.
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