Dual-licensing is when you release your project under 2 (or more) licenses at the same time.

Note that any code you want to integrate in your project, and that you do not own the copyright of, must be compatible with the 2 licenses at the same time.

You also need to be the sole copyright holder of the code if you want to pass from a single-licensing to a dual-licensing scheme.

Example copyright notice, for GNU GPL + EPL:

You may copy, modify, and redistribute this file under the terms
of the GNU General Public License version 2 (or, at your option,
any later version), and/or the Eclipse Public License version 1.0.

[standard GPL notice]

[standard EPL notice]