Sue, I think you have a basic misunderstanding of what LR does. Photo files (Raws, tiffs, jpgs, etc.) are not "in" Lightroom. LR is basically a large indexed text file, a 'database' although LR prefers to call it a catalog. The database is first of all a list of all the photos you have imported and secondly additional notes about where they are located in the computer, about what edits you want done when the image is exported into a new photo file, about what tags and keywords you want to have associated with it, about what collections it belongs to, and similar bits of information. But all this is just information about the photo file (metadata); it is not the photo itself, which sits quite independently of LR somewhere else in the computer or on an external drive connected to the computer. And the original photo file is never changed by LR, for display on your monitor LR makes its own preview images.
So you could have references to the same photo in two different catalogs and you would have double catalogs and double previews but still one photo file. It should never be necessary to import an image more than once. Remove the references from one of the catalogs or delete the whole catalog and there are no more doubles.
The process I outlined above, export as catalog/import from catalog, copies all the references to Venice-Belgium from the LR5 catalog to the LR6 catalog so that the LR6 catalog will have the same information about the same photos - wherever they may be.
