Let me see if I can break this down a little bit:
"Jeremy, There are a lot of discussions on this and yours might be the best. "
JTK - Thank you. That makes my day.
"Regarding the snippet below, what if you already have a clustered index(es) on the table you want to move?"
JTK - You would need to drop the index first and then create it in the new filegroup. Creating the index in the new filegroup will move the table and clustered index into the new file group. Keep in mind the non clustered indexes can move to the new filegroup or reside in a different filegroup depending on your goal(s).
"Will this command try to create a duplicate index?"
JTK - No - You can only have 1 clustered index per table.
"Will it drop the existing one?"
JTK - No. The DROP INDEX command removes an existing index.
"If you would just expound on the way this statement works to help me understand how it will accomplish the move I would appreciate it! Thank you, Bobby"
JTK - Sure...Let me know if this helps.