If you didn’t import the photos into an album, they appear in the Imports album in the sidebar. When all photos have been imported, disconnect the camera or device from your computer. If a message asks whether you want to delete or keep the photos on your camera after they’re imported, click Delete Items to remove the photos, or click Keep Items to keep them on the camera. Import a selection of photos: Click the photos you want to import, then click Import Selected. Import all new photos: Click Import All New Items. Import into the Photos library without specifying an album: Click the “Import to” pop-up menu, then choose Library. Import into a new album: Click the “Import to” pop-up menu, choose New Album, enter an album name, then click OK. Import into an existing album: Click the “Import to” pop-up menu, then choose the album you want from the list. To set where to import the photos, do one of the following: This option does not appear for all cameras. If you want to delete photos from the camera after importing them, select the “Delete items” checkbox. Tip: If you want Photos to open whenever you connect this device, select the Open Photos checkbox. Change where photos and videos are stored.Export photos, videos, slideshows, and memories.

Share photos using other apps and websites.

Add, remove, and edit photos in a shared album.

Change the lighting of a Portrait mode photo.

Adjust a photo’s light, exposure, and more.

View photos others have shared with you.

Use Visual Look Up to learn about a photo.

Interact with text in a photo using Live Text.

Then you can select all remaining heirs in the smart album at once and delete them with the key combination ⌘⌫

To create the smart album use 'File > New Smart Album'.

To remove the HEIC duplicates you already imported you can search for them with a smart album: If you want to keep your photos and videos in the new, space saving format, do not disable HEIC but My Photo Stream and import only by iCloud Photo Library or only by using a USB connection but do not mix import methods. If you then import photos in any other way from your iPhone, by iCloud shot Library or by connecting the device to USB, the original HEIC versions will be seen as new photos and imported as well. The most common reason for HEIC photos being duplicated as JPEGS is mixing two import methods - if you transfer your photos automatically by My Photo Stream, the HEIC photos will be transferred as a JPEG, because My Photo Stream does not support HEIC. Where are you seeing the duplicates? In your Photos Library or in some caches? Your Mac with High Sierra 10.13.6 can handle HEIC perfectly and you should be able to use the space saving HEIC format on your Mac without creating duplicate JPEGs in your Photos Library.

I asked about this at the Apple Store Genius Bar & was shown to uncheck the HEIC option to keep it from continuing to happen,

Duplicate photos mac 10.13.6