What would happen if the gametes of sexually reproducing organisms were diploid instead of haploid?

Typically, gametes are haploid cells. They only contain a portion of the chromosomes because as they intertwine with the meeting gamete the other half of the chromosome will ideally fit the other half of the cell and fertilization will occur. Moreover, when a sperm or egg cell has already a complete chromosome or is diploid no fertilization will happen.