We demonstrate a double Rabi splitting totaling 348 meV in an Ag nanocavity embedding of methylene blue (MB) dye layer. which is ascribed to the equilibrium state of monomer and dimer coexistence in MB dye. At low dye concentration. the single-mode strong coupling between the monomer exciton in MB dye and the Ag nanocavity is observed. https://jalyttlers.shop/product-category/illustrated-book/