LinkedHashSet? Does it have to be a list?

It wouldn't be too difficult to do something such as what you described, or even to create your own class that fulfills your desired functionality.

A set, particularly a HashSet, is a kind of hash map without values, so that's what you look for. A simple HashSet is enough unless you need to keep the order. In this case, follow calsign's advice.

Or, if performance isn't important, just use a simple ArrayList and if list.contains(string) returns true, don't add it.