Wikipedia
Paleopolyploidy is the result of genome duplications which occurred at least several million years ago (MYA). Such an event could either double the genome of a single species ( autopolyploidy) or combine those of two species ( allopolyploidy). Because of functional redundancy, genes are rapidly silenced or lost from the duplicated genomes. Most paleopolyploids, through evolutionary time, have lost their polyploid status through a process called diploidization, and are currently considered diploids (e.g. baker's yeast, Arabidopsis thaliana, and perhaps humans).
Paleopolyploidy is extensively studied in plant lineages. It has been found that almost all flowering plants have undergone at least one round of genome duplication at some point during their evolutionary history. Ancient genome duplications are also found in the early ancestor of vertebrates (which includes the human lineage) and another near the origin of the bony fishes. Evidence suggests that baker's yeast ( Saccharomyces cerevisiae), which has a compact genome, experienced polyploidization during its evolutionary history.
The term mesopolyploid is sometimes used for species that have undergone whole genome multiplication events (whole genome duplication, whole genome triplification, etc.) in more recent history, such as within the last 17 million years.