Two levels of recognition
The first one protects against a batch being sent twice: each file gets its own ID for the upload, so a retry after a dropped connection doesn't create a second entry. The second one compares the file content itself, so it also catches the same folder dragged in the next day.
The same file in two galleries
That's allowed on purpose. Recognition works within a single gallery, because two clients may get the same shot from a group session.
Limits of recognition
We compare the file as it reached our server, which means after your browser resized it. In practice, we reliably catch a repeat from the same computer, but the same session uploaded once from a laptop and once from a phone may end up as two entries.
A photo exported again from your editing app is a different file for Picvelo, even if it looks identical. That's not a bug, it's how comparing file content works.