Is it now impossible to import an image that is already in the catalog?

Search for the photo in LrC to find where it is. Use these four steps:

1. Click on All Photographs (its on the left under Catalog)
2. Turn off all filters (Ctrl-L once or twice)
3. Expand All Stacks (Photo->Stacking->Expand All Stacks)
4. In the Filter Bar, search for one of these photos by File name

You want to search for the actual filename while in “All photographs” instead of searching for an extension. If it is already in your catalog, it will show up. Searching for .tif should work but it is better to search for the filetype tiff in the metadata search if you want to see all tiff files in your catalog. That said, the import panel always shows the extension of files for me. Are you sure you are using Lightroom Classic? Even Lightroom Desktop/Cloudy shows the filetypes in the Add photos panel so this doesn’t even sound like Lightroom of any type.

AFAIK, nothing changed. Make sure that “Don’t import suspected duplicates” is unchecked in the import dialog. By the way: there is no new import dialog! I suspect that your import dialog is collapsed, so you just think it’s a new dialog. Is the dialog a small horizontal rectangle, as shown below? Then it is indeed collapsed. Click on the triangle in the lower left corner to expand it.