So the MM5 installer does know to close MediaMonkey if it's already running, but if it does so, it fails partway through.
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This is the error I get. It seems to usually get stuck on MMHelper.dll.
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Attempting to delete the file manually says that it's open in MediaMonkey 5 Setup.
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It appears that in this circumstance, there are two MM5 Setup background tasks running; Perhaps one of them has opened MediaMonkeyHelper.dll while the other is attempting to read it.
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I attempted to record the issue in DbgView but the setup program didn't seem to log anything to it.
It's reproducible both by running the 2266 installer when 2265 is installed+running, and by running the 2266 installer when 2266 is already installed+running.