[adrian_b]

Allah can no longer be pluralized now, but it could be pluralized in older Arabic, before the conversion to the Muslim religion.

Remnants of that time can still be seen in the Christian/Judaic Bible, where the plural of the cognate Hebrew word, i.e. "Elohim", can be encountered, though even there it has acquired in many places a singular meaning.

[throwaway2203]

my informal understanding is that Allah is a contraction of al-ilah or (the God), which was also at the time the name of a particular deity (a sort of supreme God also associated with rain). That might have to do with the associated inability to pluralize.