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Not really that I saw in my playthrough... I could be wrong, but I didn't really notice any respawning enemies or anything aside from maybe a few sections that respawn when you go in a door and come back out...and most doors close themselves at some point. If you explore and kill a lot of the enemies...and don't spend a lot on extra lives/healing...you should have plenty to fully upgrade what's available by the end of your playthrough, though.