When creating a rule, QuickBooks Online (QBO) allows you to maintain your bank's original details on its custom memo, Art. Let me provide more information about this feature.
If you tick the Also keep existing bank memo checkbox when creating rules in QBO, the original bank detail(s) will appear alongside the memo specified in your rule.

On the other hand, unticking the checkbox will remove the original bank detail(s) from the memo field, retaining the text entered on the memo box.

Moreover, this step is only specific for the rule you've created and won't add all duplicate memos. You can also check out this article for more details about this process: Set up bank rules to categorize online banking transactions in QBO.
Additionally, when it's time to reconcile your accounts, you can navigate through these resources for guidance:
Bank rules with detailed memos can provide valuable insights during financial analysis, aiding in better decision-making. Please reply to this thread if you need further help creating bank rules in QBO. We're always here for you!