Fix: Deduplicate While Preserving Order

You've inherited a small utility function, `dedupe_preserving_order`, from a teammate. Product just filed a bug: a list of a user's recently-viewed items comes back in a strange, seemingly scrambled order after duplicates are removed — even though the original list was clearly in the right order beforehand. Read the function below, find the bug, and fix it. The function should return a new list with duplicate items removed, keeping only each item's **first** occurrence, in the same relative order as the input. This is what a real "debug the codebase" round looks like: the code isn't broken in an obvious, crashing way. It runs. It even removes duplicates correctly. It just doesn't do the one thing that actually matters here — preserve order.