Check if a value is unique only when a new record is added

[DroidTeo]

Hello, 

inside the "Valid_if" check, I've added this: 

NOT(IN([RFP], Projects[RFP]))

But this controll is working on all type of events, creation and update.. 
I need to allow the users to update the record anyway.. 

So if during the creation an RFP coede is already present I cannot save it
but
if I'm updating the record, I must be able to update the other values.. 

How can I do this?

[Suvrutt_Gurjar · Accepted solution]

Please try 

ISBLANK(
  FILTER(
    "Projects",
    ([_THIS] = [RFP])
  )
  - LIST([_THISROW])
)

How to avoid duplicates is mentioned in the help article below in the end.

List expressions - AppSheet Help

[Excellere]

Thank you.  I've been trying formulas and searching for this all day.  Simple solution!  works perfectly!