Superfluously is the adverb from superfluous: doing something in a way that was not needed. It describes the manner of an action — adding, explaining, apologising, decorating — when the action added nothing. 'He apologised, superfluously, for a delay nobody had noticed.' It is rarer than the adjective, and quite formal; even fluent speakers use it sparingly. A common pattern is to drop it between commas as a small aside, the writer's quiet comment that this bit was unnecessary. That aside often carries gentle mockery, though it can also be self-directed and modest ('I explained, superfluously, what everyone already knew'). If you want a plainer everyday version, 'unnecessarily' does almost the same work and sounds far less stiff.

多餘地、不必要地（副詞）；相當於「白說了」「畫蛇添足地」那種語氣