hu-append and hu-prepend should remove duplicates when adding to a list

[stowball]

If you customise a $type variable like $hu-height-types, hu-append a list/map of per-project heights and then accidentally do it twice (for instance, by @importing the custom hu-height partial twice`, the custom classes will be generated twice.

When appending/prepending maps, this is not an issue since map-merge keeps the target if both the $source and $target contain identical keys.

While it's an edge-case (which I just ran into 😅), it could cause unnecessary confusion as to why multiple classes are output. Also, if a user accidentally overwrote 100%, for example, then their version of 100% would take precedence when hu-appending, but not when hu-prepending.