Komentarz do Powtórzonego Prawa 19:15

עד אחד ONE WITNESS [SHALL NOT RISE UP AGAINST A MAN FOR ANY INIQUITY] — This is the classic passage from which the general principle is derived that wherever the term “witness” (עד) occurs in the Torah, it means two witnesses, unless it specifically mentions in connection with it the word אחד (Sanhedrin 30a).

על פי שני עדים, “on the basis of testimony of to witnesses, etc.” The Torah here uses the number “two” in the construct mode, שני עדים, whereas the last time we had a similar verse the Torah wrote the word שנים עדים. This serves our sages as a hint that sometimes (monetary disputes) even a lone witness is not altogether ineffective but can evoke the need for the accused to swear an oath that he is not guilty of the charge brought by the litigant who only backs it up with a single acceptable witness. (Sifri)

לא יקום עד אחד באיש, “A single witness shall not stand up against any man, etc.;” this verse follows the legislation about moving boundaries to teach that if an owner of a field accuses his neighbor of having moved the mutual boundary between their fields, he has to bring no less than two witnesses to substantiate his accusation. The words of a single witness have no value as testimony, neither when matters of corporal punishment nor monetary punishment is involved.
לכל עון ולכל חטאת, “for any iniquity or for any error;” such as if the single witness testifies that the party he has seen committed a sin against the Lord, such as a violation of the Sabbath, be it intentional or unintentional.
בכל חטא, “of any sin.” This refers to sins involving only words between himself and his fellow.
אשר יחטא, this refers to sins involving money between one person and another. In all of these situations the testimony of a single witness is legally meaningless. Only the testimony of two witnesses can result in the accused being punished.
Our sages in Shevuot 40 interpret the words לכל עון ולכל חטאת restrictively, i.e. only in order to testify concerning sins committed by an action is a single witness useless; however regarding matters of oaths, the testimony of a single witness is not irrelevant at all. Example: Reuven claims that Shimon owes him a sum of money and Shimon denies it. A single witness, Levi, testifies that Shimon does owe Reuven the money. In that instance the testimony of Levi is enough for Shimon to have to repeat his denial on oath.