Data only one time in coloum

[Solandeo]

Hello,

if the operator will insert a new serialnumber the app should check that the same serialnumber doen't exist in the same coloum in a previous row. 

How can I do that? 

[Suvrutt_Gurjar]

Typically you should not user serial number generation system in AppSheet.

Sequential keys - AppSheet Help

On avoiding duplicates , please take a look at the following expression for valid_if of the column as mentioned in the article below.. However even this can fail if two users simultaneously create a / update a record.  "Use lists to validate column values"

List expressions - AppSheet Help

ISBLANK(
  FILTER(
    "Table Name",
    ([_THIS] = [Column Name])
  )
  - LIST([_THISROW])
)

[JP_Alb]

Thanks very much. It solved my problem too... 😁
Greets!