Let $c_1,c_2>0,$ and let $f,g:\mathbb{R}\rightarrow\mathbb{R}$ be functions (not assumed to be continuous) such that for all $x\in \mathbb{R}$
$$f(x+c_1)=f(x) \text{ and } g(x+c_2)=g(x).$$
Further, assume that
$$\displaystyle \lim_{x\rightarrow\infty}(f(x)-g(x))=0.$$
Then $f=g.$