Pour éviter de créer des duplicats d’abonnements RSS dans Outlook, sous File > Options > Advanced décocher:
« Any RSS Feed item that is updated appears as new »; et
« Synchronize RSS Feeds to the Common Feed List (CFL) in Windows. »

Et, pour effacer d’un coup tous les duplicats d’abonement RSS, exécuter cette macro:

Sub DeleteAllRSSFeeds()
    Dim objRSSFeedsFolder As Outlook.Folder
    Dim i As Long
    Dim objRSSFolder As Outlook.Folder
    Dim strRSSName As String
    Dim objDeletedItemsFolder As Outlook.Folder
    Dim objDeletedFolder As Outlook.Folder
 
    Set objRSSFeedsFolder = Application.Session.GetDefaultFolder(olFolderRssFeeds)
 
    'Delete All RSS Folders
    For i = objRSSFeedsFolder.Folders.Count To 1 Step -1
        Set objRSSFolder = objRSSFeedsFolder.Folders(i)
        strRSSName = objRSSFolder.Name
        objRSSFolder.Delete
 
        'Permanently Delete from "Deleted Items" Folder
        Set objDeletedItemsFolder = Application.Session.GetDefaultFolder(olFolderDeletedItems)
        For Each objDeletedFolder In objDeletedItemsFolder.Folders
            If objDeletedFolder.Name = strRSSName Then
               objDeletedFolder.Delete
            End If
        Next
    Next
End Sub

Références

https://www.outlook-tips.net/tips/outlook-duplicates-rss-items/#:~:text=You%20can%20change%20this%20setting,occur%20when%20feeds%20are%20updated
https://www.datanumen.com/blogs/2-easy-methods-batch-delete-rss-feeds-outlook/