So I followed the instllation instructions exactly, however when Im in Wyre Bash I click the Morrowind_OB.esm to activate it, I get the above message.
This was done with a fresh install of both Oblivion and Morrowind and Wyre Bash.
So I dont know where the supposed duplicates are coming from, please help

Check your Plugins.txt file
Check your Plugins.txt file in the "<user>\AppData\Local\Oblivion\" directory. You probably have a bad Plugins.txt from an old installation. Remove any duplicate entries from that file. Let us know if you're still having problems!