Answer
The internal energy for diatomic molecule is the sum of translational kinetic energy, rotational kinetic energy and vibrational energy.
So, a monatomic and diatomic gas with same temperature will have same average kinetic energy but diatomic gas will also have rotational and vibrational energy which will contribute to its internal energy.
Work Step by Step
The internal energy for diatomic molecule is the sum of translational kinetic energy, rotational kinetic energy and vibrational energy.
So, a monatomic and diatomic gas with same temperature will have same average kinetic energy but diatomic gas will also have rotational and vibrational energy which will contribute to its internal energy.