You can't really avoid uploading duplicate photos while migrating from one account to another. No matter how focused you are at the time of upload, some duplicates will always exist . Lets take an example, You went for a picnic in summers and celebrated a friend's birthday. Now there exists a possibility that there are some photos that exist in the birthday folder as well as in Summers picnic folder too.
Anyways, Picasa provides a tool that shows duplicate files in your photo collection. If you have only a few duplicates, then its easy to clean up. But cleaning large amount of duplicate photos from your photo collection is a little tricky, time consuming and cumbersome task.
If you wish to remove duplicate photos from your Picasa, then the easiest way is to use PicBackMan's De-dupe functionality. Simply associate your Picasa account and let PicBackMan's De-dupe features analyzes your complete online photo collection. It then automatically removes the duplicates & segregates them.
You can install PicBackMan's Picasa uploader for Mac from the website and start backing up photos and videos.