How is it possible that individuals with two different genotypes can have the same phenotype?7?

The phenotype is expressed as long as there is a dominant allele in the genotype, regardless of recessive allele.

eg) It is possible to have same phenotype when individuals have genotypes AA and Aa, where dominant allele is present in both. (Assuming dominant allele is A)

There is also epistasis, not sure if you are referring to that too : 3