Is it possible for organisms to possess different genotypes despite having same phenotype?
Same phenotype but different genotype is possible due to presence of dominant allele. A dominant allele , when present in genotype, always expresses itself: both in double dose and in single dose.
A person can have a dominant allele on both the homologous chromosomes (i.e. in double dose) while another person may have single dominant allele and a corresponding recessive allele.
In both the above cases the trait controlled by dominant allele will be expressed and hence there will be no distinction in phenotype.
This phenomenon is seen in all organisms including plants.
In human, dominant eye colour is expressed in both homozygous dominant and heterozygous dominant individuals.