As it is a common query and source of much confusion, we have prepared this 3 minute help file, which applies to versions 10 - 15.
A guide to loading libraries when working between different versions of ArchiCAD, including loading Migration Libraries.
Here is an example of a common incorrect library loading process:
Referencing of Libraries. Often unnecessary libraries are loaded, like AC13 Migration, AC14 Migration. Alternatively the duplication in loading of libraries, such as "ArchiCAD Library 15" & "ArchiCAD Library 15.lcf".
In addition, the most common library loading error at present, as seen on the left dialogue, is the loading of the "ArchiCAD Migration Libraries". This is a problem as this folder contains all the migration libraries AC10 - AC14. This means not only are you loading all old libraries from ArchiCAD 10 to ArchiCAD 14 but duplicating the loading of the libraries previously mentioned. This will slow down the opening of the project which is unnecessary, and can cause problems in future identification of elements used, with the resultant dreaded "duplicate loaded library" dialogue appearing endlessly in your project.
As you can see from the screenshot, we only have ArchiCAD Library 15 and ARCHICAD EXTRA LIBRARIES loaded. But you can also delete the latter if is not definitely going to be used and load it later if necessary. The fewer libraries loaded, the better all round.
After all these changes, you click OK and when checking the Library Loading Report, there will be no instances of missing or duplicated objects.