The wordsredundant, superfluous and excessive all imply that a quantity greater than the minimum amount needed has exceededsome threshold. But they all suggestdifferent implications.
Excessive indicatesthat a quantity is so high it might be a bad thing. It could be said for example, that excessive rainfall will cause a tank to overflow. If you wanted the tank to overflow to feed the plants around it, you might just call it sufficient instead.
Redundant implies that the extra abundance can be used as a means of substitution. When you buy a packet of say 100 sheets of paper, they may give you 103. The extra 3 are redundant in case only 99 of the regular sheets are provided. Animals often have organs in pairs, Lungs, kidneys etc. Its said that one kidney can perform the job of two so it that sense it is there for redundancy reasons.
If your job is to be terminated, one of the tactfulways to express this is that your position is redundant. This implies that the company found a way to do your job without you. To simply say that the person is no longer needed might imply that they can no longer do there job which is only sometimes the case and could be a harshthing to do considering their situation.
Superfluous makes no judgement on whether something is worse or better, indeed it suggests the matter is little importanceto the rest of the message being conveyed . Although the etymology of superfluous means overflown water, you would not use the term if you were concerned with wasting water or things getting wet.Instead you would say it’sexcessive. If however you were describing the motion of water in a physics class and you filled up a cup of water which overflowed into a basin you might describe the overflown water as superfluous as to not import any other concerns other than the motion of the water. In a practical scenario, saying excessive when you meant superfluous will almost always be forgiven as superfluous is a much less used word than excessive and they can both get a common point across. This would be like saying big when you meant ample. Both are matters of connotation which generally fall under fairly subjective interpretation.
As a further explanation, when acronyms are described, sometimes both the last letter and last translation of the last letter are pronounced. For example, ATM Machine, which expands to Automatic teller machine machine. Notice machine is described twice. This, I’ve noticed to be a contentious issue in communication. If you think its a good thing you wouldcall the extra M redundant. If you think its a bad thing you wouldcall it excessive. If you think its neither bad nor good you wouldcall it superfluous.
EDIT: there is also extraneous. This word is similar to excessive but works on type and not quantity. Extra objects of which the type is bad.