I think you have some misconceptions about LR.
LR only displays images "You" have imported into LR. It is not a File Manager and does not seek out images to import and display. So if you have multiples of the same image then you have either imported them twice or have those same images in different locations on your hard drive with slightly different file names and you have specifically imported them.
You say you have created your first Collection. A collection in LR is just a Grouping of images that you have already imported into LR. The images in any collection can be from multiple sources, folders on your drive or drives connected to your computer, and any image could be in multiple collections. So if you placed 100 images into a collection when you click on that collection name those 100 images will be displayed. If you then click on the All Photographs entry all images you have import into LR will be displayed along with the 100 images you have in the collection. A collection is just a grouping of images. Once you place an image in a collection it doesn't stop it from being displayed if you select All Photographs or the drive it is store on or the folder it is stored in that you have imported into LR. A collection is not a duplicate of images, it is just a Grouping of images that have already been imported into LR.
As for the slight delay that is because LR hasn't finished building the previews for all your images.
Thanks for the speedy reply
There aren’t any duplicates in the files that were imported or indeed in different locations with slightly different names on my hard drive, which is why I am confused. Particularly when I checked the box to instruct that any potential duplicates not be imported. I am aware it is not a file manager and does not go looking for images to import and display (I am not that out of touch), it is a tool for organizing, editing, and sharing your photography.
With respect to the delay, I don’t think the point has been answered. At some point LR must finish building the previews and the delay should be negated. But the delay is perpetual and significant.
If there aren't any dup images in different folders and you have the option checked "Don't import suspected duplicates" the I suggest you post a screen shot of the Library module in Grid view showing these duplicate images. Also right click on both of one of these duplicate images and select Show in Finder/Win Explorer to see the actual path to each image. Post a screen shot of the 2 Finder/Win Explorer windows showing the path.