Deduplicate an item list, and preserve order.

    For case-insensitive lists, drop items if they case-insensitively match
    a prior item.

def _dedup(items, insensitive):
    '''
    Deduplicate an item list, and preserve order.

    For case-insensitive lists, drop items if they case-insensitively match
    a prior item.
    '''
    deduped = []
    if insensitive:
        i_deduped = []
        for item in items:
            lowered = item.lower()
            if lowered not in i_deduped:
                deduped.append(item)
                i_deduped.append(lowered)
    else:
        for item in items:
            if item not in deduped:
                deduped.append(item)
    return deduped