...My opponent said that I have to copy the spell immediately before passing priority to the opponent. I said that since my original spell is still on the stack, I can target it. I'm 99% sure I'm right but need some confirmation. If anyone can quote something from the rules to support this, i'd greatly appreciate it.
Your opponent is
probably correct, depending on the exact verbal language and shortcuts involved.
Specifically, resolving the spell goes as follows:
1) Spell is declared, targets are declared, mana is paid, etc. Do all the stuffs, spell goes on the stack. You still have priority.
2) You pass priority to the opponent if you have no responses (to your own spell).
3) Your opponent passes priority back, if they have no responses.
4) Spell resolves.
This is from
608.1. Each time all players pass in succession, the spell or ability on top of the stack resolves.The key here is that you cannot take any actions after step 2) unless your opponent had a response. If your opponent also passes priority, the spell has already resolved, and it's too late to try to copy it. Also, as a standard shortcut, step 2) is always assumed after a spell goes on the stack unless
you explicitly state otherwise. You can tell, because otherwise you'd have to say "...pass priority" after every single action you take in a game, and clearly nobody does that.
As a result, if you want to copy (or counter, or whatever else) your own spell, you must state "Hold Priority" in some form, cast your first spell, and then cast your second spell in response - at which point, you verbally release priority and your opponent gets a chance to respond to the copy. If they have no responses, you regain priority and
unless you state otherwise are again assumed to be passing it back at which point they get a chance to respond to the original spell.
Here's
an example from an SCG event, where a player holds priority to counter his own spell.