Is it possible to restrict users from submitting more than one per diem expense on the same date?

Yes, you can restrict users from submitting more than one per diem expense on the same date by defining a daily limit on the number of expenses that are created under the per diem category. Here’s how:

Click Admin View on the left sidebar.
Go to Settings on the left sidebar.
(OR)
Click Settings in the top right corner of the page.
Go to Policies under Users and Control.
Select the policy for which you want to define the default per diem rate.
Go to the RULES tab.
Click + Add Rule.
Enter the rule name and select the Rule Type as Daily limit.
Define the Limit Type as Expense Count.
Enter the Expense Count as 1.
Select the Category as the default category that you’ve configured for the per diem expenses.
Click Save.