It sounds like you’re not keeping track of your in and out points within the timeline. If you have in and out points set and go to drop footage it is going to lay the footage directly over your in and out points and will do an overwrite on the existing track, or if you’re trying to start a new track it will likely do an insert.
If you’re just trying to lay down footage quickly make sure your in and out points are clear in the timeline and then drag and drop to your heart’s content. The footage will always start at the mark of your play meter. If you want to be more precise don’t drag and drop. Double click to view your footage, check your length in the timeline and then match in your left viewer before dropping it into the timeline.