Every frequency in a grouped frequency table is doubled, while the classes stay exactly as they were. The mean of the data will:

The new mean is $\frac{\sum (2f_i)x_i}{\sum (2f_i)}=\frac{2\sum f_i{x}_i}{2\sum f_i}$; the factor 2 cancels top and bottom, so the mean is unchanged. Doubling frequencies doubles the numerator, but students often forget that it doubles the denominator too. (Contrast this with adding a constant to every $x_i$, which does shift the mean.)