In theory, an Elements catalog should not contain duplicates of any file. The catalog is merely a database of files stored on your computer. If you attempt to import an exact duplicate into the catalog, it will be rejected as already being in your catalog. However, there are circumstances where duplicates will be included. In order to help you remove them, we need to know more about how the duplicates got into your catalog.
Are the duplicates contained within the same folders but have a different OS name, e.g. Photo.jpg and Photo-1.jpg?
How did you import all of these files, e.g. from files and folders, from a card reader, from a camera phone or In Bulk?
Do you have backups of your original files that have been imported into your catalog? (If you used Elements to backup your catalog, the backup files will be named with a prefix B.)
Are the files on different drives?
In the meantime, you can go to Find>By Visual Searches>Duplicate files. This search will take a while and may not be fully informative but it may help your search for duplicates. In any event, you can select multiple duplicate files using Shift-Click (for contiguous files), Ctrl-Click (for non-contiguous files) and Ctrl+Shift-Click for (multiple contiguous and non-contguous files). Once selected, hitting the Delete key will delete them from the catalog. You will also be given the option to delete them from your hard drive.
With the limited information you have provided so far, that's all the help I can give.
In that situation, it seems that the duplicates are really physically present on your phone and that for some reason, the downloader fails to detect and filter them. The normal filter is expected to keep the first found item and to ignore the duplicates in the scan of the phone. For me, the original scan searches all my photos, not only on the dedicated DCIM folder, but also on my microSD card, my Dropbox etc. So, I no longer use the default 'download' method. I prefer to select my recent photos in the DCIM folder and to move them from the Explorer to my computer before "getting" them in the catalog with the "get photos from files and folders".
Alternatively, I don't connect my phone to the computer. I use Dropbox to select the recent files from the Photos app and import them into Dropbox. Once synced with the computer, I can get files from files and folders without duplicates.