Contradiction with Malachi 4:5
Malachi 4:5 prophecies Elijah's coming before the day of the Lord, whereas Luke 7:27 attributes the coming of a different messenger to prepare the way for Jesus.
Malachi 4:5: Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:
Contradiction with John 1:21
John 1:21 has John the Baptist denying that he is Elijah, while Luke 7:27 refers to him as the messenger prophesied in Malachi, suggesting a fulfillment in John.
John 1:21: And they asked him, What then? Art thou Elias? And he saith, I am not. Art thou that prophet? And he answered, No. [that prophet: or, a prophet?]