Discount and coupon 100% off of for only 1 item in shopping cart
You could use the Flag module to get this to work, by configuring an appropriate (non-global) flag, which you should then use to (automagically) flag a user after such user applied (for the first time) the coupon code at checkout. To actually flag such users, you should use the Rules module, which integrates extremely well with the Flag module. All that's left is to find the appropriate Rules Event for such rule to be triggered (somewhere along the lines of "checkout completed" I would assume).

Any time a user tries to apply such coupon code, you can then verify if it is the very first time, or yet another time. For this verification, all you'd have to do is use a Rules Condition to check "has this user yes or no been flagged already (by the flag you created for this purpose)".

If you're not familiar with the Flag module, checkout either of these:


The set of 8 video tutorials about the Flag module.
Flag's Community documentation.