Holes in the scans - symptoms and troubleshooting

While designing the denture (from scratch or copy denture), the user can sometimes get a message “Trying to add facet to an edge as ”left" facet, but “left" facet already exists” or “Trying to add facet to an edge as ”right" facet, but “right" facet already exists”. The examples of such messages are shown in the pictures below.

Especially often ths