Dominating frequency in a "clear" 50 Hz signal
The size of $y$ is 201. So frequency bin of fft is not 1 Hz. Change code for calculating $t$:

Fs=200.
F=50.
t = [i*1./Fs for i in range(200)]


Now you have only 1 peak on 50 Hz.

P.S.


Plot magnitude spectrum
Try to modify F (49, 49.5, 49.67). Enjoy!


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