When a madd letter is followed within the same word by a consonant with shaddah, the hidden still portion of that doubled consonant creates madd lazim kalimi muthaqqal. The label “muthaqqal” refers to the doubled articulation following the long vowel.
«الضَّالِّينَ» — madd alif after dad is followed by a doubled lam. Sustain the alif for its obligatory duration and then perform the lam's shaddah distinctly.
Lengthening the alif alone is not enough if the lam loses its shaddah. Nor should the lam be overemphasized at the expense of the long vowel. Madd and consonant doubling are separate requirements that must be coordinated within the same word.