Doubled triliteral roots have identical second and third radicals. Assimilation can represent both radicals as one shaddah-bearing consonant; other environments display them separately.
RootPastPresent1st-person pastActive participle
م د دمَدَّيَمُدُّمَدَدْتُمَادٌّ
ذ ب بذَبَّيَذُبُّذَبَبْتُذَابٌّ
مَدَّ — مَدَدْتُ
Translation: He extended — I extended.
The first illustrates assimilation; the second form in the pair shows the consonants separately (fakk al-idghām). A shaddah does not reduce the root to two radicals.