It is possible if homozygous and heterozygous for a certain gene give the same phenotype. For example, if the trait for flower colour is influenced by a gene which has two alleles: the red allele which is dominant (A) and the white allele which is recessive (a), only two phenotypes are possible. Both the flowers with AA (dominant homozygous) and Aa (heterozygous) genotypes will have the red phenotype, while the plants with the aa (recessive homozygous) genotype will have the white phenotype.
It is also possible because the phenotype is not only determined by genes (genotype), but also by the environment and genotype-environment interaction. Influence of the environment can form the same phenotype in two individuals even though they don’t share the same genotype, but this phenotype cannot be inherited.
