After you have Home Sharing enabled on both computers, you can select the shared library in iTunes, and the Show dropdown menu is in the lower left corner:
The best way to avoid duplication is to use the Show menu located in the lower-left corner. Choose "Items not in my library" before transferring any content.
I recommend following the steps in the section titled "Setting up Home Sharing in iTunes" on both computers in the following article (step 3 in this article states to go to File>Devices>Turn On Home Sharing, but in the latest version of iTunes you would need to go to File>Home Sharing>Turn On Home Sharing):
If you are still having trouble after following those steps, I recommend reviewing this article:
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