I created a little tool to help remove duplicate or merged ICS files.
Move it into a folder with your 2 ICS files. Your current calendar file (You can export in 16), The ICS of the accidentally merged file.
Open up CMD and run these commands
cd <to the directory of the folder containing the exe and those 2 files.>
ics-duplicate-remover.exe <YourICSName.ics> <TheMergedICSFile.ics> <outputFileName>
EXAMPLE: ics-duplicate-remover.exe MyICS.ics Astronomy.ics MyNewICS.ics
Then you should have MyNewICS.ics, or whatever you named it. Go ahead and delete all your events in SM (Keep your backup ICS just in case) then import the new ICS file.
If you have trouble doing any of this, you could send me the files via a private message on the community and I could do it for you.