R squared when every observation appears twice

An OLS regression yields $R^2=0.56$. If each observation is duplicated, so every point appears twice, and the regression is rerun, what is the new $R^2$? Enter $-1$ if it cannot be determined.

$0.56$. The fitted line and the mean of $y$ are unchanged, so both the residual and total sums of squares exactly double, leaving their ratio and hence $R^2$ the same.