There used to be an Acquanetta - with the c to make it original - working at my local Walmart. Nice lady ... the other cashiers called her Aqua.
Then he asks why "Emma" has this pattern, but not other once popular names such as "Mildred" and "Bertha." I suspect this reflects the changing ethnic composition of the United States. Mildred and Bertha are distinctively Germanic names, and German-Americans, having often been in the U.S. for five generations or more, give their children standard names rather than ethnic ones. Then there's simply the ease and simplicity of "Emma," compared to other names that were once popular.
I think another reason many parents today want "uncommon" names for their children is that, like me, they were one of seven girls with "Common Name of 1966" in their elementary school grade, and one of nine women with "Common Name of Late 1960s" in a department at work. In my department at That Insurance Company, there were so many employees with my name that our boss called us "Mrs. Jones," "Mrs. Collins," "Mrs. Garcia." (Very dignified, actually - I kind of liked it!)
My sons have names that often make the top-10 lists - William, Thomas, Patrick, James, Daniel, Francis - but the only one who regularly finds himself in a class with duplicates is Daniel. It seems every family has a Daniel! The girls are "uncommons" - Josephine, Eleanor, Sabina, and Kathleen - because I was traumatized by Common Name 1966 ;-).