Sarah or Clara - a timeless classic.
Lydia or Livia - I adore this
Cora or Nora - not a massive fan of either though
Elaine or Elena - Elaine has negative connotations for me and is too middle aged.
Lucy or Lacey
Genevieve or Guinevere - I honestly dislike both of these, can't stand Genevieve and Guinevere is too King Arthur for me to be usable.
Gwyneth or Gwendolen - Again dislike both, must be the Gwen sound.
Freya or Fiona
Coraline or Caroline
Autumn or Summer - Again not a massive fan of either, though Autumn sounds a bit more fresh to me.