Time domain distortion vs Harmonics I have a periodic time domain sensor signal sampled at $500 \text{ Hz}$ whose amplitude spectrum I compute and obtain. From the periodicity of the signal, I was expecting the fundamental frequency to be around $f = 1.2\text{ Hz}$, and this is what I observe with amplitude say $A_1$. But besides the minuscule diminishing harmonics at integer multiples of $f$;  what may be the cause of having the harmonic at $f_2$ around $2.4 \text{ Hz}$ that has an amplitude $A_2\approx1.25A_1$  ?

The signal is of $29 \text{ sec.} \Rightarrow N = 14500 \text{ samples}$. Down is a section with a full period of the signal, the blue one. 



Below is a section of the amplitude spectrum. NFFT = 2^nextpow2(N); 
I have rectified the question.
I think this is simply a property of your signal and you indeed have two significant frequency components. I don't really know the nature of your signal, but for me, it makes perfect sense, you can distinguish two periods in your signal, as shown on the plot below:



As you can see there is period $\color{green}{T_1}$ and $\color{brown}{T_2}$ between consecutive "peaks". So by doing very rough approximations from your plot:


Corresponding frequency for $\color{green}{T_1}$ can be estimated as: $\dfrac{1}{7.1-6.3}=\dfrac{1}{0.8}=1.25 \mathtt{Hz} $
In case of $\color{brown}{T_2}$ you get: $\dfrac{1}{6.7-6.3}=\dfrac{1}{0.4}=2.5 \mathtt{Hz} $


Some quick and dirty plot of such case with two sinusoids of a given frequencies and amplitudes:



So indeed you have two frequencies present in your signal. What's more, component around $2.5 \mathtt{Hz} $ can be stronger. Please consider that your signal also has very prominent DC component (also visible at your spectrum).