Bitmap results in being unable to duplicate or exporting.

Flattened = no layers.   So of course you can't duplicate layers on flattened images.

I suspect you're struggling with a locked Background layer. 

Double click the Background layer to unlock it.

Use Ctrl/Cmd + J to duplicate it on a 2nd layer.

Hope that helps.

@Buttonhead Okay, well, I'm curious why you want or need to do all this converting in the first place but you are right that once you save as a bitmap, you won't be able to add or duplicate the layer. However, I can export a grayscaled-saved-as .bmp image without any problems. For example, I tested exporting this file to a .png and was able to open that file and work normally. Is this a workflow you've used before and now it's not working or something you are trying for the first time? What are you trying to ultimately achieve?