how to detect duplicates when importing

Photos will warn you automatically, if you try to import a photo that is already in your Photos Library.

You will see a dialog like this, if you are about to import a photo twice:

if you have duplicate photos in the folders that you are importing, try to import in smaller batches, so there are no duplicates in the same batch of photos.

It will depend on the kind of duplicates. Photos is only checking for identical files with the same file attributes. If you import different versions of the same image (different format, adjustments applied, down sized versions) Photos will regard it as a different image and import it.

And as I said, Photos will compare the new import the photos already in the library, not against the photos from the same import. If you try to import several copies of the same photo within one import session,all duplicates will be imported, because they are not yet in the library.