Generally no, but sometimes yes.

We actually know of a fairly high number animals and plants with fully duplicated genomes (tetraploidy), and most are nearly identical to their diploid ancestors. One big difference in cell size, which seems to increase with whole genome duplicates. This has few consequences, but oddly enough it affects the call frequency in tree frogs, and fruit size (and overall size) in plants. The latter has lead to deliberate selection for and creation of whole genome duplicates in crop plants, often repeatedly- strawberries are octoploid.

However, such duplications can provide "raw material" for evolution. Now you have 4 copies of a gene to work with, so you can keep the original set and let the new copies evolve. Gene duplication is more common than many people think and an important part of evolution. Whole genome duplication is less common, but has occured several times, most notably at the origin of vertebrates and the origin of teleost fishes.