WHO WAS BEING QUOTED IN HEBREWS?

Hebrews 10:5-7  Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me: In burnt offerings and sacrifices for sin thou hast had no pleasure.  Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God. (KJV) Note: the assumption of contradiction here is that the author of Hebrews attributes these words to Jesus, but they are actually said by David in Psalms 40:6-7.

However, there are many instances when David foresaw Jesus and even wrote in His voice (see Psalms 22:7-18 for one obvious example). Not a contradiction.