Make & keep
Two similar strips are not necessarily duplicates
Compare objects before deciding what a collection contains.
Nearly identical portraits can survive as different objects with different histories.
Compare complete views
Check borders, dimensions, frame order, inscriptions and the reverse of each strip. Assign separate identifiers initially. One might be a later reproduction, another might have been owned by a different person. A similar-looking digital thumbnail is not enough evidence to merge two catalogue records or discard an original.
Describe the relationship
Record your observations and keep a note of any suspected connection. If a digital copy was made from another item, describe that derivative explicitly. Preserve uncertainty about the physical objects until more evidence appears. This keeps ownership stories and annotations attached to the correct surviving strip.