ITunes Consolidation: Will it create duplicates of media files if they already exist in the new itunes destination folder but arent being referenced by the itunes library?

Question

I have a macbook pro from fall 2010. I have been using my external HD as my primary itunes folder for the past 7 yrs ( i.e it is organized by itunes default categorization, etc). However, there is some overlap between my external HD itunes folder and my comps original itunes folder (probably 20-30% of my current library's media files were already duplicated there from when i first bought the HD and transferred files to the external, however they are mostly referenced from the external now). Also there are files in the comp's itunes folder that are not on my external (probably due to organizational mishaps and podcast downloads when not connected to the external, etc). I do not know how many files this is the case for.
Recently, i decided that i wanted to consolidate all of my itunes music into one concise folder on my macbook's HD to avoid these issues going forward.  I also just bought a new external which i will use to back up my comp's itunes folder once i consolidate everything to the comp. But i plan on using the itunes folder on the comp going forward or at least using the consolidate fcn to avoid these issues.
Here is my dilemma. Prior to learning of the consolidate feature today, I changed the default itunes folder in settings to the original itunes folder on the macbook and then manually dragged all of the media files from my external itunes folder to the macbook's itunes folder. There were a ton of duplicates or overlapping folders with similar artist names, so i opted to have them merge all folders where possible. After doing this, i realized the itunes library still references these files that i dragged and dropped from the external since that is where my default itunes folder used to be. At that point i then read and learned of the consolidate itunes feature which seems like the best way to do this. But i am afraid that if i consolidate my library to my macbook's itunes folder now, will it create duplicates of the 4000 + media files i just dragged to the macbook's itunes folder which are already in the itunes library but being referenced from the old external??
Question: Is there a way that i can make these files i just dragged to the macbook's itunes folder become the ones referenced by the itunes library through the consolidate feature without creating duplicates of these 4000+ files? I want to avoid having  three copies of these files (two duplicates in the itunes and the originals on the external). Furthermore, is there a best way to find out which files in the macbook's itunes folder are not being included in the library after consolidating everything to the computer's HD? Then after that, what is the best way to start backing this all up to my new external, so that this overlapping issue doesnt happen again?  Im assuming i just need to make sure to consolidate first when changing itunes folders going forward. Thank you. I know that was a lot  to read but hopefully someone can help with this issue.

Answer 1

In general:
- The best way to check if files are in a library is to drag the whole media folder to the library.  If something was missed it will be added.  If it is in the library is won't be added a second time.
- If you have a second copy of a file and iTunes does not currently list that specific file in its database then even if it is identical to one already in the database it will add it a second time.  So if you have file xyz on the internal drive and the external drive and add them to iTunes you will end up with two entries for the same thing in iTunes because you really do have to copies of the files.  While this can happen with two copies on a single drive, it will almost definitely be the case when you have a copy of a file on two drives because iTunes really sees them as two different things.
-Unless you know how iTunes works, avoid moving files yourself.  Let the consolidate feature do all the file moving.  Manually moving files can really mess up iTunes unless you know what you are doing and have iTunes set to not try to do it itself.
- There's no clean and easy way to delete duplicates.  The best thing is to change practice so you don't create them.   Here are  references:
How to find and remove duplicate items in your iTunes library - http://support.apple.com/kb/HT2905
http://dougscripts.com/itunes/itinfo/dupin.php (commercial)
Posts by turingtest2 about different types of duplicates and techniques- https://discussions.apple.com/thread/3555601 and https://discussions.apple.com/message/16042406 (Note: The DeDuper script is for Windows)
http://www.hardcoded.net/dupeguru_me/