The Prophet's ﷺ sandal had two straps, each with a doubled thong.
حدثنا علي بن محمد حدثنا وكيع عن سفيان عن خالد الحذاء عن عبد الله بن الحارث
كان لنعل النبي ﷺ قبالان مثني شراكهما
[Sahih] Ibn Abu Shayba narrated it: 8/231, from Waki, by this route. It is also found in Shamail by Tirmidhi, no. 76, and Busiri graded it sahih. The following hadith serves as a corroborating witness for it.