The Prophet's ﷺ sandals had two straps, each with a doubled thong.
كان نعل النبي ﷺ قبالان، مثنى شراكهما.
No references are available.
Tirmidhi related it in Shamail (72), and Ibn Majah (3614), both by way of Waki, from Sufyan, from Khalid Haddan, from Abdullah bin Harith, from Ibn Abbas, who said, and then he mentioned it. Its chain of transmission is sahih. Busiri declared it sahih, and Ibn Hajar strengthened it in Fath al-Bari (10/312).