The computer needs to be running and you need to be signed into your user account. After signing into your account start Photos, so it can launch the background processes that will search for duplicates. The processes will search your Photos Library for duplicates, while you are not working with Photos. Keep Photos open, but do not interact with it. Use a different app.
If your Photos Library is on an external drive, this drive has also to remain plugged into your Mac, whenever you are signed into your user account, or Photos will lose the connection to your system photos library.
The scanning for duplicates will take a very long time, because Photos is comparing what the photos are showing. Photos is scanning for similar photos, not just exact duplicates. So Photos needs to recognize the objects in the photos and understand the scenes, not just compare the file attributes. As an example, for my library with only 50000 photos and videos at that time, it took more than three weeks, before the first duplicates have been shown. Then more and more duplicates appeared, whenever Photos came across a group of duplicates.
Which system version is installed on your Mac and where is your Photos Library stored? Photos needs full disk access - check for it in the System Settings for Security and Privacy.