SR-71
Terminator T850 model 101
The early Submarine requires Submarine Warfare, while the Destroyer requires Combustion. So they are different. It could be that in your game research of these two techs came close together.
Sorry, but you're wrong, because Early destroyer requires Submarine warfare WHICH REQUIRES COMBUSTION, the same as Destroyer. The effect is, as I said, that early Destroyer never appears, being immediately obsolete by Destroyer...
That's not a real problem for me, because i've replaced the prerequisite tech for Destroyer by another more realistic in my own xml... But I think you should reconsider this issue