carry coals to Newcastle
US /ˈkæri koʊlz tu ˈnjuːˌkæsəl/
UK /ˈkæri koʊlz tu ˈnjuːˌkæsəl/
1.
to do something that is unnecessary because there is already a lot of it available
:
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Bringing more blankets to the shelter would be like carrying coals to Newcastle; they already have plenty.
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Offering him advice on coding is like carrying coals to Newcastle; he's a renowned expert.