What is the difference in the Holy Quran between "remaining therein eternally" and "remaining therein therein forever eternally"?

"Never" (أبداً) is mentioned in the Quran when detailing recompense, whether reward or punishment, and it is omitted when there is conciseness. For example, in Surat An-Nisa, it is mentioned in verse 57, which details the recompense of believers, and omitted in verse 13, which is concise. In Surat Al-Bayyinah, "never" (أبداً) is not mentioned with reference to the disbelievers because their punishment is not detailed, whereas it is mentioned with the believers to detail their recompense. The occurrence of "abiding therein forever" (خالدين فيها أبداً) for the people of Paradise more than for the people of Hell (8 times versus 3 times) indicates the vastness of Allah's mercy.