Each newsletter integration adds its own footer button: the Sprwt Newsletter Popup (requires the Marketing Campaigns module, and only shows when Newsletter Popup - Show Behaviour is not Dont Show), MailerLite Classic (when the footer form ID and token are set), MailerLite v2 (when its form ID is set) and Klaviyo (when a form key is set). To keep a single button, either set Newsletter Popup - Show Behaviour to Dont Show or clear the unused integration keys under Business > Integrations.