dnd 5e – Can an Elder Oblex be subject to an AoE more than once?

No.

As you have observed, the Sulfurous Impersonation feature says:

For all practical purposes, the simulacrum is the oblex, meaning the oblex occupies its space and the simulacrum’s space simultaneously.

Since a spell like fireball only damage creatures once, and we treat the Oblex and its simulacrum as one creature, it is only damaged once. Blade barrier functions the same way. It can only damage a creature once per turn.