Why have two genes when one would suffice? Evolutionary pressure means that biology, unlike government, is generally intolerant of wasted effort. Therefore, when multiple genes exist presumably they are there to provide some benefit to the organism even if that benefit is not immediately obvious to us scientists. A recent report from Raghu and colleagues (Biosci. Rep. (2018) 38, pii: BSR20181690)  sheds some light on one possible reason for the existence of two Phospholipases D genes in chordates when only one is present in invertebrates.
Digital Object Identifier (DOI)
Morris, Andrew J., "Phospholipases D: Making Sense of Redundancy and Duplication" (2019). Internal Medicine Faculty Publications. 202.