def _get_prefix(concept1, concept2):
    """
    Build a  Concept UI prefix as the combination of the
    UI prefixes of two concepts.

    :param Concept concept1: The first concept.
    :param Concept concept2: The second concept.
    :returns: New prefix
    :rtype: str
    """
    # ["NMCD_NMCD", "DSLD_NHP"]
    prefixes = [concept1._prefix, concept2._prefix]
    # "NMCD_NMCD_DSLD_NHP"
    prefixes = '_'.join(prefixes)
    # {"NMCD", "DSLD", "NHP"}
    srcs = set([src for src in prefixes.split('_')])
    # "NMCD_DSLD_NHP"
    merged_prefix = '_'.join(srcs)
    return merged_prefix