PmtInfId

ISO20022 describes PmtInfId as follows:

Presence: [1..1]

Definition: Unique identification, as assigned by a sending party, to unambiguously identify the payment information group within the message.

Data Type: Max35Text, Format: maxLength: 35, minLength: 1

To conclude, the element is mandatory by the schema and it has to be unique. Uniqueness is checked by the validator unless otherwise decided with the bank