Dunno, but FCP does like to hold on to it's exports. Ever notice if you try to trash an export, FCP won't let you? You have to close the project.
But personally, I keep all my exports, good or bad until the end of a project. I just use incremental version numbers. Why? Dunno know that either. I guess I'm a hoarder.
I was having the same problem too..... trying to overwrite a file in after effects for a Final Cut Pro project.
I found that if I tabbed away from my FCP project sequence.... over to an empty sequence (or a different sequence) BEFORE trying to render my after effects clips, After effects would let me overwrite the old movie file with the new one...(saving it over the old file with the same name)
Then, (after the composition was rendered) I simply went back to FCP and "tabbed" back to the sequence for my project ....and the clip would automatically update....just like in the good old days
I guess when after effects sees the project file is open (over in FCP) it won't overwrite it until it no longer sees that FCP sequence