In all my experience of writing harmony I have never ever doubled two notes in a chord for SATB. I have been taught to triple the root, add the third and omit the fifth but not to double the root and the third (so C C E E).
If the reasoning behind it is to avoid an alto G meeting up with the previous soprano G I can't see it is necessary - as far as I know that is not an overlap. Is it a misprint then?
I might be pernickity about detail but i find small doubts upset my confidence when trying to write harmony!
Thanks for any help