It happens when you call display_module() with the same module parameter twice.
I guess you must accidentally pass that object through (one or few) another method and then call display_module on it again. Hard to say without the source.
EDIT: To clarify:
$this->display_module($module_one);
$this->display_module($module_two);
is good. Whilst:
$this->display_module($module_one);
$this->display_module($module_one);
or
$this->display_module($module_one);
$another_module->display_module($module_one);
is not good.
Kind regards,
Arek