Usually if that sort of thing happens it is because MC has found each file via two different paths. This can particularly be true if you installed MC and allowed it to "Find all your media files" by doing the scan of your PC that starts soon after your first launch MC. Most experienced users stop that automatic scan and set up MC before the first import.
For example, if you had your music in D:\Music\ and also pointed the Windows Music Library "This PC > Music" to the same location, then MC will find the files twice.
Another example, if you had Windows Shares set up, with one called \\MyPC\Music pointing to D:\Music, and also allowed MC to scan D:\Music, then MC will find the files twice.
It doesn't really matter that MC found the same files, if it was via two separate paths, if will import them twice. This is because MC allows files with exactly the same name to reside in different folders, and will still treat them as different files.
At least I think that is what has happened. It has been a long time since I did an initial import!
The best solution is probably to turn off Auto Import under "Options > Library & Folders > Run auto import in background".
Then clear the existing Library, using "the Clear Library" function available when you select the library name under Playing Now.
Then set up Auto Import to look in one or more locations for your music, but without duplication.
Then run Auto Import, and if you are happy, turn Auto Import in the background back on.
The Wiki has lots of information on Import, Library, Library Manager, and so on. Worth a thorough read:
https://wiki.jriver.com/index.php/Category:Frequently_Asked_Questions
