Question about One Definition Rule

Hello dear EE-ers,

There is a quote:

In the entire program, an object or non-inline function cannot have more than one definition; if an object or function is used, it must have exactly one definition. You can declare an object or function that is never used, in which case you don't have to provide a definition. In no event can there be more than one definition.

Please, provide example about this statement, especially regarding "in no even can there be more that one definition".

Thanks.