there are several things that can cause multiple monitors;
if your motherboard came with integrated video and someone later added a video card could cause the appearance of multiple displays, it woul'd be even more if that addon card has let's say a VGA connection AND a DVI connection. then it would show the existence of 3 displays.
Each connection would show up as a seperate display.
If that is the case where you have a motherboard with integrated video AND an actual AGP/PCI video card, you should be able to disable the on-board video by accessing your system's BIOS and looking for integrated video options for enable/disable. Doing this may also free up some RAM being used by this possible integrated video, as shared video RAM.
If this is not the case where you have any integrated video, it may be that you have a dual-head video card with VGA and/or DVI. If this is the case that you have a duallie, it does not hurt anything to have multiple showing up in your device manager. However there should be only 2 in this case.
Other than that, the only thing I may suggest is ensuring that you have the latest and correct drivers for the video system you ARE running in your system.
Maybe some more info as to kind and type of video card you are running with maybe a physical eyeball count of how many VGA/DVI connections you have on the back of your system. The VGA connections are gonna be a 15-pin either blue or black most likely. And the DVI would most likely be a white multiple pin looking thingy.