
überflüssig
unnecessary
Not necessary or not needed; more than required and therefore dispensable.
Noch ein Paar Schuhe zu kaufen, ist wirklich überflüssig.
Buying another pair of shoes is really unnecessary.
unnecessary / superfluous
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unnecessary
Not necessary or not needed; more than required and therefore dispensable.
Noch ein Paar Schuhe zu kaufen, ist wirklich überflüssig.
Buying another pair of shoes is really unnecessary.

not needed
Refers to a person who feels useless, unwanted, or not needed.
Nachdem die Jüngeren die ganze Arbeit übernommen hatten, fühlte er sich überflüssig.
After the younger people had taken over all the work, he felt he was no longer needed.
The word is derived from 'der Überfluss' (surplus, abundance). Something that is 'überflüssig' (unnecessary/superfluous) is therefore more than needed, as if it came from an abundance.
Wir haben schon genug Essen, weitere Einkäufe sind überflüssig.
We already have enough food; further shopping is unnecessary.
If 'überflüssig' (unnecessary/superfluous) refers to a thing, an action, or a remark, it means 'unnecessary'. If it refers to a person who feels that way, it means 'useless' or 'not needed'.
Eine weitere Diskussion ist überflüssig.
Another discussion is unnecessary.
The word is often used in the construction 'etwas überflüssig machen' (to make something unnecessary). This means that a new thing or situation makes an old thing unnecessary.
Die neue Brücke macht die alte Fähre komplett überflüssig.
The new bridge makes the old ferry completely unnecessary.
When a person feels unneeded, this is almost always expressed with reflexive verbs. Typical phrases are 'sich überflüssig fühlen' (to feel unneeded) or 'sich überflüssig vorkommen' (to feel like one is not needed).
Seit der neue Kollege da ist, komme ich mir manchmal überflüssig vor.
Since the new colleague arrived, I sometimes feel like I am not needed.
unnecessary
Both words can say that something is not needed. „Unnötig“ mainly denies the necessity of something, while „überflüssig“ often also suggests that there is too much of something or that it offers no further benefit. Use „unnötig“ when something is simply not required, and „überflüssig“ when it seems dispensable as an addition.

Für die kurze Besprechung wäre eine zweite Präsentation überflüssig.
A second presentation would be superfluous for the short meeting.
Für die kurze Besprechung ist eine Präsentation unnötig.
A presentation is unnecessary for the short meeting.