Simplify the following text for a Grade 12 native English-speaking person.
Original: Hubble has sometimes been incorrectly credited with discovering the Doppler shift in the spectra of galaxies , but this had already been observed earlier by Vesto Slipher , whose data Hubble used .
Simplified: Some people have incorrectly said that Hubble discovered the Doppler shift in the spectra of galaxies , but this had already been observed earlier by Vesto Slipher , whose data Hubble used .