Why a second peak at 2 KHz with same amplitude of 1KHz ?
GNU Radio is designed for working with IQ / complex signals. You are seeing the symmetric spectrum that results from treating a real-valued signal as a complex one with a zero quadrature (imaginary) component. The second peak is the mirror image of the first.
It is offset on the scale to 2 kHz because you have specified a nonzero center frequency, which offsets the frequency numbers on the scale but does not change the actual data processing.