Duplicate objects means that ArchiCAD found duplicates in your loaded libraries. Don't worry, it is just reporting it. ArchiCAD can find which loaded object belong to the object placed in your drawing.
But it is better to have none or little duplicates because it extends the loading time of your libraries. GS made subsets or migration libraries. These contain the difference between sequential libraries. Because the libraries are being developed, some object are removed or become non-compatible due to changes in the script. These objects are stored in the migration libraries.
So when migrating from 12 to 14 you should load the 14-library, plus ArchiCAD 13 Migration Library and ArchiCAD 12 Migration Library.