I need concatenate a value but not if the value already exist

[Johny_Freitas]

how the app work:
i have an app that scan somes QR’s
SO, a row (POS SPT) Scan the QR’s and another row (POS entregues) Concatenate these.

but if some one scan the same QR, my app concatenate it too.

i need a way to not concatenate if the value already exist.