Duplicating a symbol is NOT like copy and paste!
Duplicating a Flash symbol is a one-step-way to:
A duplicated symbol is not an instance.
It is a brand new symbol.
There are three, slightly different ways to do this:
One - the new symbol appeared in the Library,
And two – the instance on the stage has become an instance of the new duplicate.
Select it and check the Properties window to see.
Editing this symbol will not affect the original – they are two separate entities now.
Double click the new symbol and edit it – change something in it – and see that nothing happens to the other instances of the original.
This way you get a new symbol in the Library.
It doesn’t have an instance on the Stage yet–
drag and drop from the Library to the Stage to give it an instance (see exercise 3).
Browse the Flash FAQ for Animators
Exercise 2: Convert to Symbol + Break Apart
Exercise 3: Create Multiple Instances + Distribute to Layers
Exercise 4: Transform instances + color effects
Exercise 5: Edit Symbol + Navigate in and out of symbols
Exercise 6: Nested symbols: Sun animation
Exercise 7: Duplicate + Swap