Let $k$ and $k'$ be two keys of symmetric encryption such that for some $m$ we have $\operatorname{Enc}_k(m)=\operatorname{Enc}_{k'}(m)$. Is it possible to exist a plain text $m'$ such that $\operatorname{Enc}_k(m') \neq \operatorname{Enc}_{k'}(m')$.
In fact, is there exist a key which be equivalent only for special plain text?