if I did do it

[ostap77]

A. "We matched you DNA. You're cooked."

B. "If I did do it, she would have been lying there......"

Why would the speaker used "did do" not just "did" or " had done" in the if-clause?

[charliedeut]

A. "We matched you DNA. You're cooked."

B. "If I did do it, she would have been lying there......"

Why would the speaker used "did do" not just "did" or " had done" in the if-clause?

The speaker is doing this for stress. In this case, he/she is stressing the "if".