This conversation has been merged. Please reference the main conversation: Lightroom: No suspected duplicates when imported from catalog and some imported photos don't show in Library
Steps to reproduce:
1. Open catalog A
2. Import new photos from card reader into catalog A
3. Export some of those photos from catalog A into catalog "A-export" using "export as catalog"
4. Open catalog B
5. Import photos from "A-export" into catalog B using "import from catalog"
6. Import photos into catalog B from same card / card reader as in step 2, making sure to check "ignore suspected duplicates"
Photos already imported in step 2. should be marked as duplicates when importing from card reader into catalog B. The corresponding information should have been preserved when exporting and importing from catalog A.
Photos already imported through catalog A are not marked as duplicates and reimporting would create duplicate copies in catalog B. Some of the information available about the original file names got lost when exporting and importing from catalog A.