That's very unusual, perhaps you have two separate Apple ID accounts (and thus iCloud accounts). You can check this by clicking on each one and see what email address is listed on the right. Perhaps macOS allows you to login to the same iCloud account twice..
See if the checkmarks for what services are checked. If
Mail is checked, see if you have multiple email accounts set up in the sidebar of your
Mail application. If you don't but the mail checkmark is ticked on both accounts, it is pretty likely you can delete the duplicate account.
In theory, if they are the same account there would be no harm in deleting one of them by selecting one and clicking the delete "-" button at the bottom of that window. Although, I'm pretty sure some wise guy once said that if something works - don't touch it. Let me know how you go.