You bay be bumping the right arrow key. That increments the frames. You wouldn't notice it either. The new frames would contain the previous frames poses. To fix this get into the Anim Pallet and Add a key to all elememts for the current frame. Now drag a box around that frame. Drag the frame to frame 1. Drag a box around frame 2 and up. Delete all the other keys. To keep it from happening, Click the animation range box (Usually set to 30) and type in 1. Poser asks if you want to delete frames, answer yes. You can save this as your preferred state in Edit > Prefs...
The Nerd