Yes—using Import > Folder is the correct way to reopen an older folder in darktable, and it should not create duplicate collections just from re-importing the same folder. Darktable stores its edit metadata in .xmp sidecar files in the folder, so when you import that folder again, it simply reads that information back in.

There’s also a better in-app method for finding older work: use the Collect Images panel in lighttable, on the left side above the recent collections list. You can search by different metadata using the dropdown, such as folders, camera, or tags, then type a search term in the text box. For example, search by folder name to find a specific directory, or by camera/tag if that’s easier.

So:

• Import > Folder is safe for reopening a folder
• It should not duplicate your work just by re-reading the same images/XMP data
• Collect Images is the main UI tool for locating older collections quickly