Common ion effect is the suppression of degree of dissociation of weak electrolyte on adding a strong electrolyte having an ion common. Since, ammonium hydroxide is a weak electrolyte and ammonium chloride is a strong electrolyte having common ammonium ion, thus $NH _{4} Cl$ decreases the degree of dissociation of $NH _{4} OH$.
Hence, $NH _{4} Cl + NH _{4} OH$ exhibits common ion effect.