Or HaChaim on Genesis 47:31/אור החיים על בראשית מ״ז:ל״א

1 ויאמר השבעה לי, He said: "swear it to me, etc." Jacob felt that he needed to request an oath for the second time, that Joseph's promise was insufficient. He may have meant simply that he wanted Joseph to humour him and to phrase his promise in the form of an oath, or he may have said so without revealing his true motivation at all but making it clear that he had some reason he did not want to reveal. The reason, of course, was to give Joseph ammunition in case someone would try to prevent him from doing what he had promised. Events proved Jacob correct when Pharaoh wanted to prevent Jacob's body being moved out of the country as we have learned from Sotah 34.

2Jacob may have intended to emphasise only the word לי, "to me," meaning that Joseph should not make any mental reservations when he made this promise to his father.