Use set-backed tracking for faster duplicate argument checks

[Stephen0512]

This PR improves argument deduplication by introducing a set for faster membership checks while still maintaining a list to preserve the original argument order.

The previous implementation relied solely on a list to track seen arguments, which preserved order but required repeated O(n) membership checks. Using a set enables O(1) average-time lookups without changing existing behavior.

The issue was identified during an ongoing research project.