Every time you sign to the Lightroom Mobile and Add Images it add another on the Lightroom Desktop version, that's why it show it multiple times.
You can Create Smart Collection's by going into the collections and Drag and drop your Images from folder's, else you can select the Images first under the folder and then create a smart collection and leave box checked.
Let us know if it helps.
I think the problem is that while the folder name looks like "Imported Photos" in the Folders Panel, this is only a pretty label and the Finder/Explorer folder is actually called something else like a0ae49e570c58e5fbdefb1fd3a14b736a31654e6a505cce21d6d5612908c9820 (right click the Imported Photos and go to Explorer/Finder). It looks like smart collections haven't been updated to reflect this.
So you need the smart collection to search for the camera. In this example below, the Folder criterion should work but doesn't (that criterion is useless here). If I add iPhone 6, the smart collection doe lists every photo taken with that phone, so I'd probably change Match to All, then add extra criteria like a date range or exclude photos in normal folders by changing Folder to doesn't contain My Pictures (or wherever).