Tafsir of Al-Ahzab 33:68

"Our Lord, give them double the punishment"—that is, two punishments, each of which doubles the other: a punishment for their going astray themselves, and a punishment for leading us astray.

"And curse them with a great curse"—that is, an intense and magnified one. For "greatness" (al-kibr) is used metaphorically for magnitude, as in [the verse] "Great is the word..." Magnification is also derived from the tanween (indefinite marking). Most [scholars] have read it as "much" (katheeran) with a three-dotted tha', meaning many in number.

The invocation is introduced with the vocative [particle] repeated for emphasis in supplication and to solicit a response.