Why is the second ʿUmar in «جَاءَ عُمَرُ وَعُمَرٌ آخَرُ» nunated?

Restoring Declension: Indefinite Names, Al- and Iḍāfah

• It follows a preposition
• It denotes another unspecified bearer of that name
• It is accusative
• It follows al-