Multiple 'alternative' affiliations

[josephwright]

Currently, a case such as

\author{First Author}
    \email{email@...}
    \affiliation{First Affiliation}
    \altaffiliation{Contributed equally to this work}
    \altaffiliation{Present address: ...}
\author{Second Authors}
    \affiliation{Second Affiliation}
    \altaffiliation{Contributed equally to this work}

can't be handled correctly: the first author needs two 'personal footnotes'.