By default, Photoshop Elements should not import duplicate files from your camera. Duplicates should not even be listed in the Adobe Photo Downloader dialog unless you have the "Show Duplicates" (at the top, under your device list, next to Show) option enabled. Even if you enable Show Duplicates, all duplicate files will be unchecked.
However, it is possible that changes made to image file on the Mac may make the Organizer not believe the file is a duplicate. The Organizer looks at the file's name, size, and modified date. If a file is imported and some of this information is changed, it may no longer be matched up with the original file.
When you connect your camera to your PC or insert the memory card you will probably be prompted to select the program you want to use to import the files. If not it is probably because you have already set a default program for this task. The list of programs should include Adobe Bridge if you have installed it. Select this option to get the Adobe Bridge Photo Downloader window up.
If you don't get the option to choose to import them via Bridge you can start by opening Bridge first and then clicking on File>Get photos from camera. This will then launch this same window but initially you won't see any images. You will need to browse for the right drive in the 'Get photos from' drop down box top left. Once you have selected the drive all your images should appear.
By default all the images should have a little tick underneath them. If you do not want to upload all the images you can use this box to deselect those you don't want.
On the right there are various options. First the Save Options. Here you select where you want your images to go and can specify a folder name. You can also request the files are renamed.