Merge all music into iTunes
To import music copy it to the folder \iTunes Media\Automatically Add to iTunes

Apparently there is no native bulk deduplication functionality.

One can identify duplicates within iTunes by going to View -&gt; Show Duplicates.

However to bulk deduplicate presently one needs to find third-party utilities.  Most are not free.

I tried the shareware iTunes Duplicate Song Manager, but not only did it miss about half of the dupes found by commercial programs, but it also failed to remove them from the iTunes index when it removed the files.  For $12.95 Dupe Away managed to clean everything up and find the other obvious duplicates.