For most workflows, sharpen once and do it with the final output in mind.

If you shoot RAW, in-camera sharpening usually does not affect the RAW file; it mainly affects JPEGs. If you shoot JPEG in-camera, that sharpening can be applied there.

For RAW editing, Lightroom is usually the best place to handle sharpening because it gives you the most control. The Develop module is generally preferred over relying on printer sharpening. Lightroom export sharpening is more of a simple convenience option, often useful for quick web or email output, but you generally wouldn’t want to stack Develop sharpening, export sharpening, and printer sharpening all together.

Yes—multiple sharpening stages can effectively over-sharpen the image, which may emphasize grain/noise and create harsh edges.

A practical approach:

• RAW: disable or ignore in-camera sharpening
• Do your main sharpening in Lightroom
• Leave printer sharpening off unless you have a specific reason to use it
• Use export sharpening only as a targeted output step, not in addition to heavy sharpening elsewhere

Also remember that sharpening should be adjusted for the destination: web images and prints often need different amounts.