The cold water line downstream from the water heater is siphoning water from the water heater causing the cold water to be warm when it comes out of the faucet. There is a "T" that feeds the water heater and then keeps going downstream to a few outlets. If I turn off the valve to the heater I get cold water downstream. When I turn off the hot water supply valves at the downstream faucets I still have warm water. It's not just one downstream outlet that has this problem, it's everyone. Also if I turn on a hot water faucet upstream to the heater I get cold water at the downstream faucets.
Is there such a thing as a one way valve going to the water heater to prevent this siphon?

You need to install a heat trap which is a one way valve. Some heater come with nipples but most do not. This is a hardware store item or home center.