  The $S_8/\sigma_8$ tension in the large scale structure can be explained by
decaying dark matter with an almost degenerate spectrum and small enough decay
width. Here we propose the Gemini dark matter model, which contains a heavy
mother particle $\chi_3$ and two twins $\chi_{1/2}$ which are almost degenerate
in mass and are produced at the same time. The dark sector is charged under the
same Froggatt-Nielsen symmetry that can explain the hierarchy of the Standard
model Yukawa couplings. The slightly heavier $\chi_2$ decays into $\chi_1$ and
the axionic component of the flavon, which washes out the small scale structure
and resolves $S_8/\sigma_8$ tension. We present the production mechanism of
Gemini dark matter and viable parameter regions. We find that despite the
preferred dark matter mass being $\mathcal{O}(1)$--$\mathcal{O}(100)$ keV, they
constitute cold dark matter. The Gemini dark matter model predicts an abundance
of dark radiation that will be probed in future measurements of the CMB.
