A copying skill doesn't seem that useful in a 1 vs 1 duel. The guy you copy the skills from will be more proficient at using them than you after all
I don't think that's always the case in absolute.
You can compensate the lack of experience with more imagination than your opponent. A versatile thinker might make better use of a skill than one who just applied its most direct effect.
But overall yes, this advantages the original holder of the skill as they had more time to practice and test the limits of the skills.
Other factors might come into play like base stats or gear.
If both holders have the same strength multiplier skill, the one with the higher base strength will have the advantage.
And it would make sense for someone who held such a skill for a long time to build up their base strength to optimize its effect.
Same thing with combining a sword skill with a rare sword.
Randomly copying skills isn't as good as making a properly thought-out build with multiple synergies.
Copying might have its uses, but rarely is it worth simply mirroring your immediate opponent.