We demonstrate a simple and unique double-Brillouin-frequency spaced multiwavelength structure. It uses the reflection property of an isolator and fiber loop mirror which is made of a 50:50 coupler. This scheme has only one loop, instead of two or more loops, to separate the odd and even Stokes waves generated by Brillouin scattering. The light going into loop cavity can be absorbed twice to produce a Brillouin Stokes wave twice in one round loop, so a double-Brillouin-frequency spaced multiwavelength harmonics wave can be achieved in the output of the structure. Finally, our experiment successfully achieves a double-Brillouin-frequency space of about 0.1475 and 0.1734 nm in two different type structures.