Translation

My first thought is to say
"Sum qui sum"
but this is also used in the Latin Vulgate to mean
"I am who am"
so on second thought perhaps
"Ego sum qui ego sum"
for clarity.

trifidus, and other Latinists, if you are reading this, what's your opinion?

All OK.

it might be "sum quis sum"; it's different than in Vulgate, and you don't have to use twice "ego"

or

"ego sum, qui sum"