In Dash, you cannot delete any customer accounts but you can merge an account into another. When doing this any invoices, team history, etc will be merged into the chosen account.
From the customer profile page that you would like to keep, click the 3 dots to the right of Tasks and select "Merge Customer" from the dropdown menu.
Any duplicate accounts will appear below. Select the one that you would like to merge with and press the "Merge Selected Account into Primary" button.