  The definition of dual fusion frame presents technical problems related to
the domain of the synthesis operator. The notion commonly used is the analogous
to the canonical dual frame. Here a new concept of dual is studied in
infinite-dimensional separable Hilbert spaces. It extends the commonly used
notion and overcomes these technical difficulties. We show that with this
definition in many cases dual fusion frames behave similar to dual frames. We
exhibit examples of non-canonical dual fusion frames.
