Find an example of a partial order set with a maximal element that is not unique.

My question here is, since a maximal element is define as:

m is a max of a set X if $\displaystyle m \leq x $, then x = m.

Not unique means that there are at least two, but how can I find two such elements? If we have two different max, say m and n, then either $\displaystyle m \leq n $ or $\displaystyle n \leq m $ since $\displaystyle m \neq n $, but then by definition of a maximal element, m is n...

Thank you!