Given, $f(x)=f(2x)$ evaluate $\int_{-1}^1{f(f(x))}dx$

Question

Let $f:\mathbb{R}\to \mathbb{R}$ be a continuous function and $f(x)=f(2x)$ is true $\forall{x\in{\mathbb{R}}}$. If $f(1)=3$, then the value of $$\int_{-1}^{1}f(f(x))dx$$ equals.

I don't have any idea how to proceed. Any hints would be helpful.

Thank you.