What is the chance of winning twice when the odds are 50 percent?

If you mean like a coin toss, it's 50% probability of winning, also known as even or 1:1 odds.

Winning once has a probability of p=0.5. Winning twice has a probability of p*p = p^2 = 0.25 = 25% = 1/4 = "1 chance in 4" = "odds of 3 to 1 against."