Need help on Valid if function to avoid duplicates

Hey guys, I have created an app to keep date and time records of water that I have to buy here.
So in my form, there are 3 physical columns and they are given below.
Date/Time ( Type : DateTime)(initial value - NOW())
Description( Enumlist - front tank, Back tank)
Key column (Hidden)(App formula - =TODAY()&"_"&[Description])

So if the user makes an entry for today for the front tank again today, back tank again, both tanks are allowed but duplicate entries of any of the above during today is not allowed.
7/7/2021_Front tank √
7/7/2021_Back tank √
7/7/2021_Front tank, Back tank √
7/7/2021_Front tank X

I tried but nothing comes to my mind. Please help me with writing the Vaild if formula and which column to place it. If I have to use any virtual columns in this case. Thanks guys.

You may wish to take a look at the last part of the article referred to below. The part that describes an expression to avoid duplicates. You essentially need a similar expression further limited by dates rather than avoiding duplicates in the entire table. So you could add date condition in the filter.