There are actually ETFs that contain themselves (or rather, ETF X is e.g. 10% ETF Y which is 10% ETF X. So long as the total amount of X owned per unit of X is

There are actually ETFs that contain themselves (or rather, ETF X is e.g. 10% ETF Y which is 10% ETF X. So long as the total amount of X owned per unit of X is less than 1 (in absolute value), the arithmetic mostly works out. Though I don’t know if any ETFs directly contain themselves.