my little confusion. this is possible only if i have more money in the wallet correct ?
No. Even if the address would be empty after the first transaction confirms, it is still possible to attempt a double spend.
The key issue here is that you are making two transactions using
the exact same bitcoin. It would be like if I have a $20 note in my wallet, and nothing else. I can still promise that $20 note to two different people, but only one of them is going to get it.