(Exodus 21:17) states: "And if one curses his father and his mother, he shall be put to death." The Mekhilta asks why this verse is needed at all, since (Leviticus 20:9) already says: "Every man who curses his father or his mother shall be put to death."
The answer lies in a single word. The Leviticus verse says "every man" — using the Hebrew word "ish," which specifically denotes an adult male. From that verse alone, we would know only that a man who curses his parents is liable for the death penalty. But what about a woman who curses her parents?
The Exodus verse fills this gap. It says "if one curses" — using a gender-neutral formulation that encompasses both men and women. By stating the prohibition a second time with broader language, the Torah ensures that the law applies equally regardless of the offender's sex.
This kind of analysis is characteristic of the Mekhilta. What appears to be a simple repetition between two books of the Torah turns out to serve a precise legal function. The Leviticus verse establishes the principle and the punishment. The Exodus verse extends the scope. Neither verse is redundant. Each carries information the other lacks. Together they construct a complete law: any person — man or woman — who curses their parents faces the ultimate penalty. The Torah needed both verses to cover both halves of humanity.