A subject that I've thought about extensively a couple of times now, to be perfectly honest.
Here we go.
60s: Jane Birkin and Marianne Faithfull
https://i.imgur.com/A5eo4Ss.jpg70s: Grace Slick and Debby Harry
https://i.imgur.com/dTko1KH.jpg80s: Belinda Carlisle and Susannah Hoffs
https://i.imgur.com/bSjWdvi.jpg90s: Aaliyah, Natalie Imbruglia, Mariah Carey (pre-plastic surgery) and Shania Twain
https://i.imgur.com/uKMZ9h6.jpg00s: Faith Hill and Nelly Furtado
https://i.imgur.com/wz0p2iD.jpgPresent day: Cheryl Cole, Katharine McPhee and Nora Arnezeder (yes, two pics for Nora, for obvious reasons)
https://i.imgur.com/jBPlxW5.jpgAll time: Jewel Kilcher
https://i.imgur.com/4UB0SEb.jpg