Let $X$ to be an irreducible cubic surface in $\mathbb{P}^3_{\mathbb{C}}$. Is it true that if $\dim(\operatorname{Sing}(X))\geq 1$, then a line is contained in $\operatorname{Sing}(X)$? I.e, does a cubic irreducible surface with a singular curve has a double line?
This may be a silly question, and already proved, but I'm searching the literature for an answer to it. It is not clear to me if it is true. Furthermore, I am not sure how would attempt to answer this question myself.