Delivery Xpress Admin Dashboard: Restrict and show error message when two same tip-Amount entered

Describe the bug
Duplicate Tip Amounts should not be Allowed and display some Error Message. The application currently allows users to enter the same tip amount more than once. There is no validation or error message shown when duplicate tip values are entered, which may lead to confusion or incorrect data submission.

To Reproduce
Steps to reproduce the behavior:

Navigate to the tip entry section.
Enter a tip amount (e.g., 10) in the first input field.
Enter the same amount (10) in another input field.
Observe that both entries are accepted without any error message.
Expected behavior
The system should detect duplicate tip amounts and restrict the user from entering the same value more than once. An appropriate error message should be displayed, such as:
"Duplicate tip amounts are not allowed."

Screenshots

Image
Desktop (please complete the following information):

OS: [e.g. Windows]
Browser [e.g. chrome]
Version [e.g. Latest]

Start by reproducing the issue in the admin dashboard's tip entry section: enter 10 in two fields and observe that both values are accepted. Locate the JavaScript code responsible for tip input validation, then verify that duplicate values are rejected with an error message and cannot be submitted.

Written by the indexing model from the issue text.