Duo is correct (in most cases) and you are also correct (but only in very rare cases). The issue is, that the german word describes a group of things (and its also possible that this group of things is just one single thing). Yet theres a plural form for certain situations.
I try to make it clear with examples, just for the case that my explanation wasnt enough.
We assume that you have a computer and stuff like a mouse, keyboard, headset and so on. In english you will say something like "I'm selling a computer with accessories (plural)". To say the same in german, you have to say "Ich verkaufe einen Computer mit Zubehör (singular)". In this scenario you will always use the singular form.
Now there are rare cases where you can use a pluralform in german. The accessories of the cars are totally different. Die Zubehöre der Autos sind völlig unterschiedlich. This is a possible way to say it, but again to use a Plural of Zubehör is very rare (and you might even argue if it has a plural, but at least duden.de agrees with me that it has one).
Even in such situations you will see more often either "Das Zubehör der Autos ist völlig unterschiedlich." or "Die Zubehörteile der Autos sind völlig unterschiedlich.". All these 3 sentence basically say the same, but have slightly different connotations (which dont matter in 99% of the cases). So for my examples these are the very small differences. Zubehöre states that they are different groups for different cars. Zubehör states that there are different parts of a group (this does not always mean a lot, you could always interpret the group as group of ALL existing accessoires). Zubehörteile states that they are just different pieces, now we do not talk about it as group instead were talking about the single part and then we create a plural.
As a useful analogy, the word "equipment" is never used in the plural form in English. In this case, it's also the same in German (die Ausrüstung).
On the other hand, "information" is also never plural in English, but is in German (die Informationen).
Which words don't have a plural form is not always the same between languages, unfortunately.
Never say never. In English, the equipments store would be a valid usage, if various kinds of equipment were to be sold there. Similarly, "fish" is a plural if all the fish are of one type, but fishes CAN be used if there are various types being sold in a pet store. Nevertheless, even a native speaker might look askance at you if you dared to speak correct English here...
Using your example of computer accessories: when you want to say that one of the computer accessories needs to be replaced, but you're not yet certain which one it is, how do you distinguish?
Please don't tell me "by pointing or naming each one individually" that is cheating & might not be possible in some situations.
All of the computer accessories are das Zubehör but so is each individual accessory when it comes to testing them. How do i avoid giving the impression that all of das Zubehör need to be replaced?
equipment and accessories are two very different nouns! I speak of my necklace and bracelet as accessories to my dress, and I think of them as plural. An accessory, i.e. one accessory would be just the bracelet or just the necklace. I've read MortiBird's explanation but do not find it logical.
My Collins English-German dictionary seems to say that Zubehor (with the umlaut) can be a masc or neuter noun. Presumably regional differences any thoughts on this ?
Duden agrees: https://www.duden.de/rechtschreibung/Zubehoer
Substantiv, Neutrum, oder Substantiv, maskulin
However, I've never heard der Zubehör myself.
And further down it says das, seltener auch: der Zubehör, calling the masculine form "rarer".