I've ended up with quite a few extras in sizes I use a lot. For example, 8, 10, and 12mm. I once happened on a sale of decent but cheap 10mm wrenches and bought four as raw material for building special tools. I have several 8mm wrenches in assorted styles because I deal with a lot of metric brake bleeders, so it's often very helpful to try more than one to find the specimen that seems to fit the best, or happens to allow the best "swing" in the available area.
In general, I have and regularly use metric sets of:
- Regular ol' combo wrenches
- Ratcheting combos (plus extras in 8, 10, and 12mm)
- Flex ratcheting combos
- "Mountain" style looooong flex ratcheting combos
- Gearwrench "X-Beam"; these have sort of a twist in the beam. Very comfy, long, and very useful option in some situations.
- Deep offsets
- Stubby combos
- Capri thin wrenches (VERY nice for the money!)
- One flare nut wrench (a 10 + 12mm covers everything I work on)
- Probably something I'm forgetting...
I keep a full no-skip set of regular combos in my main box, but for the other sets I have the sizes I hardly ever touch in a "storage" box out of my way. Everything I work on is Japanese (or Korean), so I keep the JIS stuff, 8, 10, 12, 14, 17, 19mm, etc., available, and move the 9, 11, 13, 15, 16, 18mm etc. to storage. This saves a LOT of space and time in daily use (like, if I need a 12 or 14mm, I don't even have to look), and the less-used sizes are still easily available if I do need them.
I also consolidated my useless inch **** into one drawer way in the back last year for those rare lawn mower occasions, and donated all the un-used duplicates and extras. This freed up a lot of space. Useless inch **** really seems to build up over the years because so many sets include both for some damfool reason.
YMMV, of course, depending on what you normally work on, but the point is that you can have a lot more options on hand if you can cull or separate as needed to fit your work.