Like carrying coals to Newcastle.

The comparison describes bringing something to a place that already has plenty of it. The effort is unnecessary because the destination has no need of what is being offered.

It can also describe a well-intentioned contribution that is useful in general but redundant in a particular setting. The problem is not the value of the thing itself, but where it is being taken.

Use it to question the point of bringing goods, skills, or help to a setting already well supplied with them. It can make that judgment without suggesting that the offering has no value elsewhere.