This should work I think:
=If( Match( logmonth , $(=Chr(39) & GetFieldSelections(DueMonth,Chr(39)&','&Chr(39),12) & Chr(39) ) ), logmonth )
The entire expression between $( and the next to last ) will be evaluated and the result will be put into the outer expression verbatim and then the rest of the outer expression will be evaluated.
So if you have selected January, March and June in DueMonth this expression:
Chr(39) & GetFieldSelections(DueMonth), Chr(39) & ',' & Chr(39) , 12) & Chr(39)
will evaluate to this string:
and this will due to the $-sign expansion be put in so the outer expression will become:
=If( Match( logmonth , 'January','March','June' ), logmonth )
Which will return a true value (a positive number) if there are any matches.