Fourier transform with periodicity at the harmonic frequency
If you have two periodic functions $f(t)$ and $g(t)$, where $f(t)$ has fundamental frequency $f_0$ and $g(t)$ has fundamental frequency $2f_0$, then their sum $h(t)=f(t)+g(t)$ is periodic with fundamental frequency $f_0$. Without any further knowledge about $f(t)$ or $g(t)$ there is no way to separate the two, because if the only knowledge you have about $f(t)$ is that it is periodic with fundamental frequency $f_0$, then there is no way to distinguish it from the sum function $h(t)$, which has the same periodicity.