That's because your tracks have different <Album Artist> values.
- Go into the entry displaying the |
- Right-click the track > open the tag editor
- Remove the | on the tag to make it uniform with the other value if that's how you want it.
https://musicbee.fandom.com/wiki/Getting_StartedI'll also embed your screenshot here for ease of reference:

Edit:
I noticed that in my collection, a number of tracks show up twice. But the only difference seems to be a '|' after the second artist listing.
What causes this?
Not sarcastically speaking, but you actually did answer your own question you know.
Also, it's not that the same track(s) are being duplicated inside MusicBee. It means that you have two copies of that same song on your system.
Right-click each one > search > locate in windows explorer to find the actual 'duplicate' file that's making the same track show up twice.