Thank you dartho for the fast reply
UPDATE: Deleting duplicate login code from the "register" and "register_rules" templates solved the problem.
Regarding duplicate login boxes...
When I search the lightweight skin templates for this piece of code...
...I can find it in 3 templates:
1) navbar
2) register
3) register_rules
My other custom skins have the code only inside the navbar template.
Regarding to guest/ unregistered user posting... we use the vB Human Verificatin Manager (random question to prevent spam).