In an artwork of a pokemon, Lunala in English, it is written ルナアーラ. Why, after the ナ, are there both an ア and a ー? It would just be pronounced as if any of those two were removed, so why are they both there?
It would just be pronounced as if any of those two were removed
No, that's not true. Japanese people will pronounce ルナラ, ルナーラ and ルナアーラ very differently. ルナラ is 3 morae long, ルナーラ is 4 morae long, and ルナアーラ is 5 morae long. ルナアーラ would be approximated as "lunar, ah, rah." Forget how it has been translated in other languages for now.
This type of "double-elongated vowel" is uncommon, but can be seen in compound words whose element happens to contain an elongated vowel. ルナアーラ is probably etymologically ルナ (lunar, "moon") followed by アーラ (ala, "wing"), and is pronounced as such.
A similar example in native Japanese would be 大鬼 ("big ogre"), which is written as おおおに in hiragana. This お is three times as long as normal お. A great-aunt (parent's aunt) is 大叔母 (おおおば) in Japanese, and is pronounced very differently from 叔母 (おば, "aunt") or 大葉 (おおば, "big leaf").
Bonus: Some speakers may unconsciously use a glottal stop when saying words and phrases like ルナアーラ, 大叔母, いい色 or 王を追おう to indicate the second part is a separate component (although the glottal stop is usually not a distinguish feature in Japanese). Instead of a long continuous vowel, you may hear a slight "stop" between ルナ and アーラ, depending on the speaker.