def get_common_form(placeholders):
    """
    Extracts the common target form from a list of scalars while making sure
    that the given targets are equivalent.

    Args:
        placeholders: Placeholders with possibly differing target forms.

    Return:
        str: Common target form.
    """
    target_form = None
    for member in placeholders["scalars"]:
        if target_form is None:
            target_form = member.target_form
        elif member.target_form is not None:
            if target_form != member.target_form:
                raise ValueError("Variant target forms provided for "
                                 "single entry.")
            target_form = member.target_form

    return target_form