Is Easy Duplicate Finder 7 for Mac Catalina safe to download?

Dump the download. If you're not downloading from the App Store then only ever download from the maker's site. Otherwise you have no idea what the app may have been infected with. You can google EDF 7 and find the maker's website.

Unfortunately it's impossible for any of us to certify is any app is safe or otherwise. However, if you tell us why you think you need to use it, perhaps we can suggest better known apps that do the same thing. And yes, I know you're looking for duplicates... but duplicate what?

You want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the following apps with these results:

PhotoSweeper - $9.99 - Demo version available. 

PowerPhotos - $29.95  

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited versions, along with their metadata between libraries.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.

I've found both to be safe, leaving the final decision to delete or not up to the user.

Then you definitely do NOT want to use this app on your Photos library. it doesn't advertise itself as understanding the internal structure of the library. It could wreak havoc.

I would use this

https://www.fatcatsoftware.com/powerphotos/

for the task.

Are these photos managed by Photos.app or any other photo manager? Using an app on such a collection that is not certified for that will cause dataloss.