Say, I send 10 BTC from my account (holding 20 BTC) ABC to a vendor's account XYZ. Right after that, I send 10 BTC from my account ABC to the same vendor's account XYZ (consider I'd like to buy 2 products). We now have transaction t1 and transaction t2, which seem identical.

Besides the practical point of creating 1 transaction holding 20 BTC from ABC to XYZ, can t1 and t2 occur in one (valid) block? If so, why doesn't the malicious vendor not re-submit t1 infinite times untill my account has been depleted?