This is usually normal behavior caused by a flash mode or assist function, not bad batteries.

Common causes mentioned:

• red-eye reduction: this intentionally fires a few pre-flashes before the exposure
• autofocus assist in low light: the flash may emit low-level pulses to help the camera focus
• nikon cls/commander or certain i-ttl settings: these can trigger pre-flashes used for metering or wireless control

What to check:

1. Look for red-eye reduction flash mode and turn it off if enabled.
2. Test in brighter light. If it stops, it may be AF-assist behavior.
3. Check whether the flash or camera is set to commander/CLS mode.
4. Review the flash mode on the camera and SB-800 to make sure it’s in the standard mode you expect.

If the final exposure flash still fires normally, the issue is most likely a setting rather than a hardware fault.