Well I've noticed in my own family history research that after generations of children all called Peter George William John and Henry, Sarah Mary Ann Jane Elizabeth; around the late 1800s/ early 1900s suddenly different names were coming in. All the names in fact that are fashionable for babies now- Florence, Matilda, Flora, May plus boys names like Victor, Percy and Stanley.
But there have always been fashions in names. My mums friends (b 30s-40s) were all called Brenda, Barbara, Margaret, Betty, Valerie and Patricia. The girls I went to school with (b 60s) were called Tracy, Sharon, Karen, Julie, Kim, Claire and Alison. My DD1s friends (b 80s) were Kayleigh, Kirsty, Chloe, Jodie, Samantha, Hannah. DD2s friends (b 2007) are Lily, Molly, Tilly, Maia, Ellie, Emily.
You'd no more have found a Brenda at our school than a Karen at DD1s.