What happens if your father is a judge? The Torah prohibits cursing judges: "Elohim you shall not curse" (Exodus 22:27). It also prohibits cursing leaders: "And a prince in your people you shall not curse." If your father holds one of these positions, he is already protected by the specific prohibition against cursing officials. Where, then, does the separate prohibition against cursing a parent come in?
The Mekhilta constructs a logical argument. A judge is not the same as a prince, and a prince is not the same as a judge. Each has a different type of authority. But there is a common element between them: both are "in your people" — members of the community of Israel — and you are exhorted against cursing them. The prohibition protects them because of their communal standing.
Now apply this to your father. Your father is also "in your people." If the Torah protects judges and princes from being cursed because they belong to the community, then your father — who also belongs to the community — should receive the same protection. But the Torah goes further: it provides a separate, independent prohibition against cursing parents, establishing that this offense is its own category, not merely a subset of cursing officials.
The result is that cursing a father who is also a judge violates multiple prohibitions simultaneously. Each one generates independent liability. The Mekhilta treats the overlapping protections not as redundancies but as separate legal obligations that compound when they converge in a single person.