Arabic words are mostly made of up 3 root letters. When the last two root letters are the same, this is a kind of irregularity called مضاعف. The basic pattern that مضاعف takes is that these two ending letters will appear as one letter with a ّ on it. For example, the word يَضُرُّ (meaning – he harms) has ض ر ر as its root letters. ر will have a shaddah on it with a harakah. Irregularity occurs when you want to put a sukoon (ْ ) on the last letter. So, when there is a need to make the last letter with a sukoon, the verb can take on 3 different forms. For example, in the forbidding form, the verb has to have a sukoon on the last letter.
In order to forbid, “Don’t harm!” there are 3 versions of it :- لاَ تُضآرَّ لاَ تُضآرِّ لاَ تُضآرِرْ
The first two versions are just the shaddah with either a fatha or a kasrah, and the third version splits up the last two root letters and so does away with the shaddah, so the sukoon is able to be on the last letter.
In Ayat Ad Dayn (ayat of debt) there is an occurrence of this where this grammatical detail shows us the beauty of the Quran and the Arabic language.
Al Baqarah 2:282
O you who have believed, when you contract a debt for a specified term, write it down. And let a scribe write [it] between you in justice. Let no scribe refuse to write as Allah has taught him. So let him write and let the one who has the obligation dictate. And let him fear Allah , his Lord, and not leave anything out of it. But if the one who has the obligation is of limited understanding or weak or unable to dictate himself, then let his guardian dictate in justice. And bring to witness two witnesses from among your men. And if there are not two men [available], then a man and two women from those whom you accept as witnesses – so that if one of the women errs, then the other can remind her. And let not the witnesses refuse when they are called upon. And do not be [too] weary to write it, whether it is small or large, for its [specified] term. That is more just in the sight of Allah and stronger as evidence and more likely to prevent doubt between you, except when it is an immediate transaction which you conduct among yourselves. For [then] there is no blame upon you if you do not write it. And take witnesses when you conclude a contract. Let no scribe be harmed or any witness. For if you do so, indeed, it is [grave] disobedience in you. And fear Allah . And Allah teaches you. And Allah is Knowing of all things.