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.

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.
necessary
'nötig' and 'überflüssig' are opposites when it comes to whether something is needed. 'nötig' means that something is needed for a goal. 'überflüssig' means that something is not needed or is more than would be necessary. 'überflüssig' often sounds a little stronger than 'unnötig' because it also says: this can go, or it could have been avoided. If something is really needed, it is 'nötig'; if it has no benefit, it is 'überflüssig'.

Die zweite Kopie des Formulars ist überflüssig.
The second copy of the form is unnecessary.
Für die Wanderung im Park ist morgen festes Schuhwerk nötig.
Sturdy shoes are necessary for the walk in the park tomorrow.