使用mysqli时如何显示重复错误

I am using the code below to input transactions into a database. Each transaction has a unique "id" associated with it, so in phpMyAdmin, I declared the "id" field (actually called transaction_id in my database) to be a unique value, thus preventing duplicates. Now when I download the transaction data into the database, it only adds in new transactions.I am using the code below to input transactions