كلا

كِلَا • (kilā) m (feminine كِلْتَا‎ (kiltā))

1. both, each (of two)
   • 609–632 CE, Qur'an, 17:23:

     ‏وَقَضَىٰ رَبُّكَ أَلَّا تَعْبُدُوٓا۟ إِلَّآ إِيَّاهُ وَبِٱلْوَٰلِدَيْنِ إِحْسَٰنًا ۚ إِمَّا يَبْلُغَنَّ عِندَكَ ٱلْكِبَرَ أَحَدُهُمَآ أَوْ كِلَاهُمَا فَلَا تَقُل لَّهُمَآ أُفٍّ وَلَا تَنْهَرْهُمَا وَقُل لَّهُمَا قَوْلًا كَرِيمًا‎

     wa-qaḍā rabbuka ʾallā taʿbudū ʾillā ʾiyyāhu wa-bi-l-walidayni ʾiḥsanan ʾimmā yabluḡanna ʿindaka l-kibara ʾaḥaduhumā ʾaw kilāhumā fa-lā taqul lahumā ʾuffin wa-lā tanharhumā wa-qul lahumā qawlan karīman

     And your Lord has decreed that you not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [so much as], "uff," and do not repel them but speak to them a noble word.

   • 609–632 CE, Qur'an, 18:33:

     ‏كِلْتَا ٱلْجَنَّتَيْنِ ءَاتَتْ أُكُلَهَا وَلَمْ تَظْلِم مِّنْهُ شَيْـًۭٔا ۚ وَفَجَّرْنَا خِلَلَهُمَا نَهَرًۭا‎

     kiltā l-jannatayni ʾātat ʾukulahā wa-lam taẓlim minhu šayʾan wa-fajjarnā ḵilalahumā naharan

     Each of the two gardens produced its fruit and did not fall short thereof in anything. And We caused to gush forth within them a river.

• كِلَا‎ (kilā) inflects for gender to match the noun or pronoun it modifies. It only inflects for case when followed by a pronoun; when followed by a noun, it is always in the nominative case regardless of syntax. The following noun or pronoun is always in the genitive dual (an idafa construction). Thus
  ‏بِكِلْتَا يَدَيْهِ‎ ― bikiltā yadayhi ― "with both his hands"
  has the nominative form كِلْتَا‎ (kiltā), despite the syntax calling for a genitive, but
  ‏بِكِلْتَيْهِمَا‎ ― bikiltayhimā ― "with both of them (fem.)"
  uses the genitive form كِلْتَيْ‎ (kiltay) in the same syntactic context.

كَلَّا • (kallā)

1. no way, never, by no means, absolutely not

   ‏كَلَّا ثُمَّ كَلَّا‎ ― kallā ṯumma kallā ― no and no again, a thousand times no, over my dead body, not on my life

• This word is often used when one answers one’s own rhetorical question:

‏أَفَهٰذَا تَنَاقُضٌ؟ كَلَّا، بَلْ إنَّهُ تَأْكِيدٌ‎

ʾafahāḏā tanāquḍun? kallā, bal ʾinnahu taʾkīdun

So is this a contradiction? Not at all. It's a confirmation.