Incomplete Migration from iPhoto to Photos

I suspect some of my photos in iPhoto have not migrated to Photos. If I reimport them, (there may be a large number) will I get duplicates? Will the original date data be preserved, or will the dates be reset to the time I did the second import?
Are there other tools I should use to investigate my old iPhoto holdings? I have been snooping around my libraries by means of “show package contents”