Is it possible for two people to have the same phenotype but different genotypes ?

Same phenotype but different genotype is possible due to presence of dominant allele

Explanation:

A person can have a dominant allele on both the homologous chromosomes  while another person may have single dominant allele and a corresponding recessive allele.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.