John Thurber III had two daughters named 'Mary'?

John Thurber III m. Mary Brown 8 Apr 1728 Swansea, Bristol County, MA.
His 2nd child, b. 1729/30, Mary Thurber m. James Sisson 21 July 1751.
His last child, b. 1749, also shown as Mary Thurber m. John Harvey 3 Oct 1771.

It seems unusual to name a second child with the name of an older child still living...Does anyone know the story of how this came to pass?

Kind regards,
W. Babcock