I ended up drilling down to a lower level folder on the left side of the import screen, rather than staying in the top level folder, and the duplicates disappeared. I do have the avoid importing duplicates option checkmarked, so the duplicates were grayed out, but I'd never had that before. I don't think that's your issue here (but something to keep in mind shooting different bodies), I also this past week had a similar instance in LR, with images showing twice. I started adding dates to my naming system to account for this. But when I had my very first camera, I did once end up that I had the same file numbers for several images spread out over a year because the numbers roll back.
You can also import with serial numbers or other unique metadata to prevent this from happening as well. It also helps me easily identify which camera was used, without having to check the metadata. So my D700 imports files starting with 2021_700_xxxx, my Z6 comes in at 2021_Z6_xxxx, and my Z6ii comes in at 2021_Z62_xxxx. I have changed the default numbering in my cameras to show the body name upon import so I can keep unique names regardless of frame number (which rollover every 9,999 images). This is usually not an issue if you are filing images in different folders, but it can happen.
HDR Efex Pro 2 is accessible in Lightroom under File>Export with Preset.Are the numbered images different, or actual duplicates? If you are shooting with two different cameras, it's theoretically possible that the default camera names are the same for both (ie, both cameras happen to have rolled to the same image count number. It is handled a different way than the other plug-ins. Please note, you do NOT want to do this with HDR Efex Pro 2. The plug-in(s) you added should now be accessible. You may add another plug-in by going back and following steps 6 - 10 again, otherwise close preferences by clicking OK.
Typically it can be found at either, C:\Program Files\DxO\Nik Collection\Viveza 2\Viveza 2.exe, OR Macintosh HD |Applications | Nik Collection | Viveza 2 | Viveza 2. Midway down on the right click on the "Choose" button and locate the Nik Collection application file for your product.Click on the "External Editing" tab at the top of preferences.While Lightroom is still open, click Edit> Preferences OR Lightroom > Preferences.Lightroom does not need sidecar files Lightroom sidecar file is named. If it does not appear to be installed, please continue to step 4. Import all images (all raw format) Reject unsharp and broken images and delete.
If the filters do not appear in Lightroom then a manual installation may be necessary.
Un-checking this box will make all of the plug-ins appear in the Photo > Edit In menu. This option is in Edit > Preferences > Presets OR Lightroom > Preferences > Presets. This seems to occur because the “Store Presets with Catalog” option in Lightroom's Preferences is checked.
Occasionally, after running a Nik Collection installer, the software may not be installed correctly in Adobe Lightroom.