This is where measurements by multiple groups come in. I would point out that in 2019 the basis for the SI base units were changed because of issues with the standards. The international community - lead primarialy by NIST and their counterpart agencies in other countries noted that, among other standards, that measurements of the "standard meter" bars were changing measureably and the replicates of the "official bars" differed from one another.

They proposed and agreed upon new "standards" based upon measurments with better precision and accuracy. See the Wikipedia summary [1]

[1] https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba...