Alright, let's break it down into an equation.

1 question has 4 choices.

Assuming that only one is right that would give you a 25% chance of being correct if you randomly chose an answer.

If 2 of the answers are the same, and are both the correct answer, then 2/4 of the choices are correct. So picking an answer at random would leave you with a 50% chance of correctly answering the question.

Simple.