File already exists in the catalog

There has been many discussion about this from the first versions of Elements. No complete description of the principle has been available but the main criteria are known:

- the suspected duplicates must have the same size in pixels

- the 'data_taken' should be exactly the same

- usually the file name is irrelevant (exception: you use the 'duplicate' command).

Both criterias above are stored in the current catalog and each import item is checked with the content of the current catalog.

Those criteria are excellent for photos from cameras, less so for scans where the date_taken is generally irrelevant.

Edit:

To add to Greg's  correct answer (we were writing at the same time) it may happen that you are importing from part of a folder tree (not from camera) and that there are duplicates (according to downloader criteria) in various subfolders. The comparison of criteria happens for each file, compared to the content of the catalog including the previous imported duplicate. Only the first imported member of the duplicates will be accepted.

When you are in doubt whether the error messages means a corrupted file or a rejected duplicate, try again the import into an empty test catalog. If you want to know what are the suspected duplicates in the catalog, create a 'saved search' with the date and size criteria which will return those files. Visual comparison will give you the explanation.

I don't know the exact answer to your question, but here are my observations. 

• I import via the Adobe Photo Downloader from a memory card,
• I shoot in jpg and raw, 
•   I have  automatic stacking of the jpg/raw files turned on,

If I  use the same memory card to import new photos, without deleting the photos already imported from the card, to the same folder(s) as the original import, then the APD will consider the top image in the stack as already existing in the catalog, but will import the hidden image in the stack again.  Because a file of the same name already exists in the folder, the OS will append the -1 to the name.

Is this anything close to what you are experiencing?