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opportunity; occasion; chance
買い物のついでに郵便局に寄った。
I stopped by the post office while I was out shopping.
ついでにこちらの書類も確認してください。
While you're at it, please also check these documents.
散歩のついでに公園でコーヒーを買った。
I bought coffee at the park while I was out for a walk.