In Arabic repeating a word is simply a confirmation or an emphasized confirmation that intents to clear any doubt about the importance of what is said or to make clear that this is meant and nothing else.
Here a definition of التّوكيد (at-Tawakyd: confirmation) in Arabic grammar (translation by google translate):
هو أسلوب لُغويّ يؤتى به للتّحقيق وإزالة التّجوّز في الكلام، أي أنّ فائدته هي تمكين المعنى في نفس المخاطب وتقويته، وإزالة ما علق في نفسه من شكوك، وإماطة ما خالفه من شبهات.
It is a linguistic method that is brought about to investigate and remove obscurantism in speech, meaning that its benefit is to enable meaning in the same addressee and strengthen it, and to remove suspicions that hung in himself, and to convey suspicions that contradict it.
For example you may find these verses in the qur'an where such a word (or even sentence) repetition occurs:
قَالَ فَالْحَقُّ وَالْحَقَّ أَقُولُ
[ Allah ] said, "The truth [is My oath], and the truth I say - (38:84)
أَوْلَىٰ لَكَ فَأَوْلَى . ثُمَّ أَوْلَىٰ لَكَ فَأَوْلَىٰ
Woe to you, and woe! (34) Then woe to you, and woe! (74:34-35)
كَلَّا سَيَعْلَمُونَ . ثُمَّ كَلَّا سَيَعْلَمُونَ
No! They are going to know.(4) Then, no! They are going to know. (78:4-5)
كَلَّا إِذَا دُكَّتِ الْأَرْضُ دَكًّا دَكًّا
No! When the earth has been leveled - pounded and crushed - (89:21)
Another form of such confirmation with emphasize is a confirmation by synonyms or by terms that dissolve any doubt of what is meant:
فَسَجَدَ الْمَلَائِكَةُ كُلُّهُمْ أَجْمَعُونَ
So the angels prostrated - all of them entirely, (see 15:30 and 38:73)
These are the main types of emphasized confirmation.
In Arabic grammar we speak of التوكيد المعنوي in the second case and التوكيد اللفظي in the first case.
In the example you've listed the point is just that instead of a one by one repetition as it is the case for my first examples the confirmation comes by a word of the same root or an attribute that explains the word and confirms it like if you said:
an obscure darkness or an obscure/dark night
At night it is dark and darkness certainly is obscure this simply confirms the meaning and excludes any other possibilities like for example moonlight or a shimmer that might lift the nights darkness.
Or like for example
The poem of a poet
It is clear that a poet should be known to have poems nevertheless you may attribute a specific poem to a poet to exclude a wrong reference or meaning.
This is certainly from a grammatical point of view a deviation from the above rules, but the intention and impression left is the same. Therefore one can't call it confirmation التوكيد as in the cases shown above and would correctly call it a description الوصف as ibn 'Ashur did it for (25:22) to make the distinction between both kinds of expression. However a description can be a confirmation as it makes things clearer and lifts any doubt.
Note that this also appears in form a repetition by a sentence like in the hadith:
إِنَّ اللَّهَ لاَ يَنْزِعُ الْعِلْمَ بَعْدَ أَنْ أَعْطَاهُمُوهُ انْتِزَاعًا..
Inna Allaha laa yanzi'u al-'ilma ba'ada an 'a'taahomooho intizaa'an..
I heard the Prophet (ﷺ) saying, "Allah will not deprive you of knowledge after he has given it to you, but it will be taken away through the death of the religious learned men with their knowledge. Then there will remain ignorant people who, when consulted, will give verdicts according to their opinions whereby they will mislead others and go astray."
Here the translator did not translate the literal meaning or was not able to save the meaning without changing the phrase as it would rather be translated (my own translation):
"Allah will not deprive you of knowledge by (sudden) deprivation,rather it will be taken ...
An interesting article on this topic in the context of hadith texts in first place can be found -in Arabic- on islamweb.