Yes, the PCV vaccines are considered “one and done” for vaccination against the serotypes covered. However, if a new PCV vaccine covers additional serotypes, someone previously vaccinated with an older PCV vaccine may still want the newer one to cover the additional serotypes.
Someone who wants maximum coverage from current vaccines may want to get both PCV21 and PCV20, but current guidelines recommend PCV21 or PCV20 (meaning exclusive-or), not both, so convincing providers (and insurance if you want insurance to pay) may not be that easy.