compacting a mutable sequence

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i am looking for some already existing implementation of this logic i am ready to implement for myself, if necessary. i want to "compact" a mutable sequence by removing duplicates, keeping the order of both the unique items and each first item of duplicates. converting to a set loses the original order, so it is not an option to convert to a set and convert back, but maybe a set can be used to help detect duplicates in an implementation. anyone know of an existing implementation?