Validation Rule - 2 Records not have the Date/times

I have an object that is running several equations based on past data. It is a tiered commission structure where once someone hits a certain point in payments then the commission doesn't apply. I've got all the formulas working; however, I'm running into an issue. If someone makes a mistake and needs to go back and correct something then the results for the equations wouldn't be accurate any longer. This can be fixed by using the sum field with a filter of before the current time.

Here's my problem. How can I make sure that no two records have the exact same time? I was hoping for a validation rule that would tell someone to change the time but that isn't an option.