Strongly recommend that you reconsider using repeating fields.
Suggest instead that you have a related table set up - linked to your Contacts table with a unique ID:
Contacts----------<Invoices (meaning one-to-many)
Invoice::fkContactID = Contacts::pkContactID
then your Invoices table will have data such as Inv Number, Date etc. and a portal on Contacts can display all invoices for that Contact in date order..