This happens when you assign a Client Contact as the Billing Contact in Core, but the contact does not have the Company name field filled in. So Core uses the contact's name in the first line and the contact's Display As name in the second line, which is typically the client name by default.
To resolve this, enter the company name of the client or client contact, which then appears on the second line of the invoice. If you do not want to keep the company name in the second line, then remove the contact and company name, and only keep the Display As name. The invoice will show only the Display As name.