If the rule 'Use memo on invoices' is checked on multiple phases of a project, its phased invoice displays multiple copies of the message to appear on the invoice.
To resolve this issue, you should remove the rule 'Use memo on invoices' from all the phases being invoiced. Instead, the 'Message on Invoice' text should be set on the Projects > Settings > Billing Options tab for the parent project to print on all invoices. If a message is defined here, it shows up automatically on the invoices.