Hello,

although this is not a problem because indeed it's not a duplicated URL but only the same URL repeated twice, this could happen if you have:

1. The same article linked to a menu and included through the data source 'Content'
2. you have used the precaching to generate the sitemap

The solutions to have only 1 URL:

1. Exclude that article from the 'Content' data source and leave only the one included from the menu with priority '90%'
2. Don't use precaching for the sitemap and doubled links will be discarded

Cheers,
John