Clarification Needed: Power Spectral Density of Real Signal
This stems from Parseval's theorem. For any signal, $\int_{-\infty}^{+\infty}|x(t)|^{2}\mathrm dt = \int_{-\infty}^{+\infty}|X(f)|^{2}\mathrm df$ and hence for a real signal, the one sided PSD must be multiplied by a factor of 2 because of Symmetry. Intuitively, the Fourier Analysis of any signal provides us with a spectrum which has Negative frequencies too because of the fact that Sines and Cosines can be constructed using Euler's theorem of using Complex Exponentials of positive and negative frequencies; and it so happens for any real signal that all you need are complex exponentials which are conjugates of each other to construct them. 

The negative portion is not exactly "redundant". The signal still owns those "negative" frequencies and hence anything present there accounts to its energy.