What's the problem with this exactly? Doubling synonyms is a standard way many languages construct words.
- Pathway (path + way)
- Courtyard (court + yard)
- Safeguard (safe + guard)
- Nonetheless (none + less)
And it's not just English:
https://websites.umich.edu/~duanmu/Diachronica99.pdf
Group and set are not complete synonyms. A group is a number of things. A set implies that there is some connection between the things. If you have a group of silverware, it means you have a bunch of spoons in a drawer. If you have a set of silverware, it means you have a curated collection of spoons that were manufactured to be used together.
A groupset is a number of things that have a connection to each other, meant to operate as a complete system.
Yes. My point, though, was that "group" (as in, e.g., "Shimano 105 group") was the commonly used term until "groupset" began appearing. Why replace a perfectly adequate term with a newer one that means exactly the same but is redundant in at least two senses? No one would argue that "Shimano 105 group" and "Shimano 105 groupset" differ in meaning, I hope.
(And, as you must have noticed when you typed your reply, the Bike Forums spellchecker agrees with me in rejecting the neologism "groupset" but is perfectly happy with the four counter-examples you offered above.)
Then a poster pointed out that the newer term probably arose in Great Britain, where "chainset" was already in use. That explained that.