|I suspect it's just a case of you not understanding the settings or the instructions, or both.|
Go into the camera menu and disable the built-in flash. That's what I do.
If that doesn't apply to yours, read the manuals carefully again.
I'm assuming the external flash is a Nikon unit same make as camera? If it's not it may not be wholly compatible.
Nikon equipment is usually very reliable so it's unlikely to be faulty.
message edited by phil22