Provision for handling equal values when asserting collection order with NUnit

Scenario:

I am using Assert.That(values, Is.Ordered, Descending) where values is a list of DateTimes in string format.

Expected outcome:

Sequential values that have the same string value are treated as valid by the ordering function.

Actual outcome:

Duplicates/sequential values with the same value are not allowed and an exception is thrown.

Is there a built in option/parameter that would allow me to specify that equal/duplicate values are ok?