Channels missing when selecting for Channel Collections

I have a custom source and I'm trying to add some channels from this source into Channel Collections. However, several of the channels from this source aren't there to select from the list when I try to manually add channels to the collections. The channels show up fine when I Manage Lineup for the source. They show up in the Guide and I'm able to play the channels just fine. But I'm unable to add them to a Collection.

I've tried removing and re-adding the source. I've tried restarting the DVR server. I've tried using different browsers to configure the DVR server. Nothing works.

EDIT: I think it might have something to do with the channel names being duplicates. Is there a setting for that on the server? I know you can disable stacking duplicates on the client, but I'm not sure how to do that on the server so I can build out my collections.

I think it might have something to do with the channel names being duplicates. Is there a setting for that on the server? I know you can disable stacking duplicates on the client, but I'm not sure how to do that on the server
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You haven't specified, but based on your issues I am drawing the assumption that some (all?) of these sources are custom channels (read, internet-based M3U playlists); is this the case? If so, have you following the requirements of custom channels and ensured that item had a unique channel-id tag in the playlist?

(Yes, Channels supports external IPTV playlists; but there are restrictions for compatibility.)