Where is the video ending up longer than the original....at import on the Timeline or in the Timeline content exported to a file saved to the computer hard drive or other?
What are the properties of the video involved...
interlaced or progressive
pixel aspect ratio
What is the brand/model/settings for the camera that recorded this video?
What computer operating system is involved, and what video card/graphics card does it use?
Let us start here and then decide what next.