Doubly Free Software is a term coined in the Oekonux project. It is used to distinguish from Single Free Software.
All Free Software guarantees the basic freedoms of Free Software for the users. While Single Free Software is limited to those user freedoms in Doubly Free Software the producers of the software are also free.
The freedom of the producers is mainly expressed by not being forced to produce - be it by being paid to do a job or be it to complete a diploma or similar. In other words: Producers in Doubly Free Software Projects contribute because they want to, because they feel their contribution is part of their Selbstentfaltung.
Contributors to Single Free Software may be forced by alienated reasons to contribute for example by being paid by an employer.