Try resetting the library:
Press Windows key + R
Type: control
Hit Enter
Open Troubleshooting
Open Programs
Open Windows Media Player Library
Complete the wizard, restart then check if its working.
If not...
You can try uninstall then reinstall it to see if it helps.
Press Windows key + R
Type: optionalfeatures.exe
Hit Enter
Expand Media Features
Uncheck Media Player
Click OK
Restart if prompted
Press Windows key + R
Type: optionalfeatures.exe
Hit Enter
Expand Media Features
Check Media Player
Click OK
Restart if prompted
Check which folders Media Player is monitoring.
Click the Organize menu > Manage libraries > Music
Select any additional folders that might have the same tracks and remove them.