Start making use of duplicates to record alternative sources.

This will produce resources like

{
    "type": "WebPage",
    "url": "http://gdk.gdi-de.org/gdi-de/srv/ger/catalog.search#/metadata/d7255b1c-1e3f-41e6-9aeb-e8f3464cb567",
    "description": "Alternative Quelle geodatenkatalog"
}

which will also enable us to better understand the results of the deduplication process.