Yes, I have many more duplicates for my RF system than my DSLR, in part because I'm more interested in the rendering characteristics of various RF lens eras, but also for practical reasons, such as speed, size, technical quality. RF lenses are also very small compared to typical modern DSLR glass, so having multiple copies, even concurrently in the camera bag during an outing, doesn't feel as burdensome and offers options I would not bother with when using the DSLR.
For example:
3 at 21mm:
21 Lux for speed and rendering
21 SEM for size and absolute sharpness/technical quality
ZM21/2.8 because it was the first 21 for RF I bought and am too lazy to sell it. It has since been replaced in day to day use by the SEM, though is very similar in capability by f/5.6.
I also bought the CV21/1.8 to pit against the 21 Lux. In some ways it outperformed the Lux (field curvature characteristics), but didn't sway me in respect to rendering, so sold it. One of the few RF lenses I've actually sold.
I'm a fan of 21mm and there are times I carry the Lux and SEM at the same time to leverage the strengths of each.
Multiple at 35mm. Actually, I use 28mm a lot more, but only one at that focal length - 28 Cron (but am toying with adding a 'vintage' option; either an older 28 Elmarit or Canon LTM). 35mm is a FL I use sporadically and over the years tried to fit into my daily kit.
ZM35/2.8 - first RF lens I bought. All around very nice and tiny, but sometimes too slow
CV35/1.2 II - fast lens, decent rendering, but huge and its field curvature annoys me when trying to get two or more people in focus at wider aperture settings. I've found I prefer fast 50s more than fast 35s, so now rarely use this lens and should probably sell it. If I want a fast 35, the ZM35/1.4 seems like the better alternative now, but would really like to find a bargain 35 Lux pre-ASPH for its rendering and size.
Canon 35/2 LTM for rendering and size
CV35/1.7 M - my current go-to 35mm. Very sharp, pleasing rendering, works great on digital along with my default modern M glass kit.
50mm.... too many to list (in the range of 8-10). Most are older and acquired for rendering characteristics, but they're often also relatively inexpensive and make easy impulse purchases. Of these, the favorite is a Zeiss Opton f/1.5 Sonnar, particularly for people photos (a close second would be the CV40/1.4, though maybe it should be in the 35mm category). I've always liked the look of the 50 Cron Rigid and finally recently found one at a reasonable price, but haven't shot much with it. My main go-to 50 is the Lux ASPH for the combination of speed, performance, rendering. It has some quirks that annoy me, but from my research only the 50 Cron APO would address these. Unfortunately its cost is a high obstacle for reasons practical and psychological.
~90mm:
90AA for technical performance - sharpness and modern rendering
Nikkor 10.5cm f/2.5 LTM for 'vintage' rendering.
Sold a 90/2.5 Summarit for the AA. In some ways I regret this, since the Summarit was smaller, lighter and less complicated in regular use. The AA is technically better, but the improvement is incremental rather than considerable. For several years the AA was the 'what if' always sitting at the back of my mind at this FL, so had to try it when I found one at a good price...
The 90 I now want to try is the 90/4 macro because a lot of my 90 use is daytime urban and natural landscapes, often stopped down a fair amount. Here the wide open performance and rendering of the AA is irrelevant and I'm just lugging around more lens than I need for f/8 or f/11.
After all this, the lens combo I usually take with me is 21/28/50/90 comprised of the SEM, Cron, Lux ASPH and AA. Lately I've also added the CV35/1.7 M.
Why keep so many lenses? Other than practical reasons for specific applications: for fun.
