Shaddah signals a doubled consonantal articulation: the first part is unvowelled and the second is vowelled. Sukun indicates that no separate short vowel follows the consonant.
رَبِّ
My Lord!
Its بّ is doubled; reducing it to an ordinary single b fails to preserve the prescribed form.
قُلْ
Say.
Here the lam has sukun and must not acquire an invented vowel. Shaddah and sukun matter for accurate reading just as the short vowels do.
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Foundations of Tajweed, Recitation Errors and Opening Etiquette
Shaddah signals a doubled consonantal articulation: the first part is unvowelled and the second is vowelled. Sukun indicates that no separate short vowel follows the consonant.
رَبِّ
My Lord!
Its بّ is doubled; reducing it to an ordinary single b fails to preserve the prescribed form.
قُلْ
Say.
Here the lam has sukun and must not acquire an invented vowel. Shaddah and sukun matter for accurate reading just as the short vowels do.