Burst Photos are usually those taken on a modern iPhone in "Burst Mode" where you depress the "shutter" button and hold it down longer than usual - which is quite easy to do inadvertently.
So first question: were these photos originally taken on an iPhone?
The error message can mean different things. It may mean that the "burst" photos have actually been moved to the Photos app and that repeatedly trying to import them again tells that you cannot import what is now a Duplicate.
So just check in your Photos library (ie open the Photos app) and see if they are there - they may seem to appear as a single image, but on closer inspection, you can just make out the edges of the pictures superimposed on each other.
If that is not the case, and they are definitely not there -
One workaround assumes the pictures in question are still on your iPhone: open this Link and scroll to the very bottom, where someone has posted how to solve this problem -
https://discussions.apple.com/thread/8337543
If they are not still on the iPhone and only on the Desktop, try to identify the "Burst" photos and move them to a separate Folder. Then try to import them from there using the Photos app ie - open Photos > File > Import and navigate to that Folder.
Please post back.
Ian