双節棍[nunchaku] nunchaku (two linked fighting sticks)
一双[issou] pair (esp. of folding screens)
一卵性双生児[ichiranseisouseiji] identical twins
双紙[soushi] written work (esp. a bound text, as opposed to a scroll)
自己双対[jikosoutsui] self dual

双刃[moroha] double-edged
双暗車[souansha] twin propellers
双殻類[soukakurui] bivalves
双眼[sougan] both eyes

双璧[souheki] matchless things
完璧[kanpeki] perfect
完璧さ[kanpekisa] perfection

圭璧[keiheki] ritual jades worn by feudal lords in ancient China
パー璧[paapeki] perfect
完璧主義[kanpekishugi] perfectionism (psychology)