How to highlight a row as duplicate as soon i enter duplicate value

Let's Say i need to copy a 5 digit number based on the value i see in external url. I copy that value in excel. There might be chances that i get the same 5 digit number in another different url. Now if i enter this in next blank row, it should highlight as a duplicate. Reason that 5digit number has a hyperlink in the url which i need to open and copy the email id in excel right next to the 5 digit number. If i enter a duplicate 5 digit number, that row should get highlighted with red or something so that i do not need enter the email again in next column instead i leave it blank and fill it together later by selecting blank and using few other functions which i'm aware of. how can i achieve this. Conditional format can used only after i copy all the values. I want this to be proactive so that i dont need to copy email every time for a same 5 digit number. Sorry for my english, i tried my best to explain the situation.

Let me try in another way. I have 5 unique numbers from A1-A5. Let's say i enter the value present in A2 to A6. Now A6 should get highlighted with color or something to show that it is duplicate.

Originally Posted by dosydos

I don't understand why you said you can't use conditional formatting. You can do conditional formatting for cells you haven't used yet.

Thanks this worked for me. You saved my day.

This worked for single digits. I tried for huge numbers, it did not work.
501240154
503996338
504180679
505315345
505370382
these are 5 examples. i couldn't replicate your formula. it was not highlighting. Can you check once?

Sorry for the spam. I now realized that this highlight will work only of i paste as a value.

I tried to copy paste the value present in another excel. it didn't work. it worked only after i pasted it as value.