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We do indeed have only one complete manuscript of the Gospel of Thomas. It was found in 1945 in a group of books known as the Nag Hammadi library, and is written in Coptic.
However after its discovery three known papyrus fragments, from the collection known as the Oxyrhynchus Papyri, and thought to be unconnected collections of sayings of Jesus, were in fact portions of the Gospel of Thomas, although the Greek versions are noticeably different from the Coptic. The fragments date from the second century.