Actually, there can be good reasons, which involve detailed requirements. For instance:
-Army develops armored vehicle
-Marines need armored vehicle, get Army vehicle
-Army vehicle doesn't meet Navy safety requirements
+Army stores HE shells in bunkers waaay far away for everything else on their bases
+Navy ships not that big (and "sunk due to magazine explosion" is a common theme in naval warfare), so safety is bigger deal
+Marines must use Navy ships for amphibious capability
-Marines make different armored vehicle (add ability to float as well)
-Congress / people claim waste because two different vehicles built for large $$$ that look / appear to do the same job.
This definitely isn't always true, but it's probably true more often than the layperson expects.
This argument can be done vice versa if you include that the Army doesn't want to pay for Marine-specific features or needs something that can be procured and logistically supplied / maintained in much larger numbers.