Boolean double negation
Double negation is the idea that if we have a NOT of a NOT, we can simplify it to just the thing itself. So, for example,
\overline{\overline{A}} = A
This works with larger expressions, too. For example:
\overline{\overline{A + B}} = A + B \overline{\overline{A \cdot B}} = A \cdot B \overline{\overline{\overline{A} + \overline{B}}} = \overline{A} + \overline{B}
This is sometimes also called the double complement law.