def string_from_list(l):
    """
    Construct a string from a list of strings
    :param l: list of strings
    :return: string containing elements of list l separated by a comma
    """
    s = l[0]
    if len(l) > 1:
        for i in range(len(l)-1):
            # only add this string to the list if it is different from the previous strings
            e = l[i+1]
            if e not in l[0:i+1]:
                s += ',\t\t' + e
    return s