outputs to dual monitor problem

[fartonmyear]

when i open my laptop screen it outputs to both the laptop and external monitor even if i'm not using an external monitor. my resolution is 1024x768 but it thinks i have an externam monitor with 1280x1024 and so my laptop monitor shows more than it can, if that makes any sense. how do i fix this. it happens whenever i close my laptop and open the lid.