Check for non-unique multi-index

[michaelmckinsey1]

From ensemble.py:

except ValueError as e:
    estr = str(e)
    if estr == "cannot handle a non-unique multi-index!":
        warnings.warn(
            "Non-unique multi-index for DataFrame in _fill_perfdata. Cannot Fill missing rows.",
            RuntimeWarning,
        )
    else:
        raise

As stated from a previous comment, instead of checking the error string, we should check if the index is non-unique.