You can't have two transactions with the same tx id in the blockchain, as tx id is calculated based on transaction contents. If that was possible you would be able to send the same transaction twice, meaning you would spend the same coins twice and double the balance of the receiving address.
There actually are multiple transactions in the blockchain which have the same txid. These transactions are coinbase transactions included in blocks prior to the deployment of BIP 30 and BIP 34 (and were the impetus for BIPs 30 and 34). Only the most recent transaction of a transaction with duplicate txids is the one that is considered to be spent from and included in the UTXO set. The other transactions with the same txid have provably burned coins as they are unspendable. BIPs 30 and 34 have since disallowed duplicate txids and made it nearly impossible for duplicate txids to occur (except when there are hash collisions).