In order to avoid errors during content/edit using the Page Builder, we need to limit the number of field using ezlandingpage to one.

There is already a function for this in the kernel:

/** 
* Indicates if the field definition of this type can appear only once in the same ContentType. 
* 
* @return bool 
*/ 
public function isSingular() 
{ 
    return false; 
}