I'd say the most handy thing to have two of are the common or basic wrench sizes. I usually find myself needing two at once more than one alone.
wrenches is something you want 2 of each, I have craftsman RP and Pros. Good to have when breaking rustily nuts and bolts.
I have one set of combo wrenches in metric and sae in standard length. What would be a good compliment to them.
Combination wrenches; two of each size.

on the two sets of wrenches. I started with two sets of craftsman RP std length combination, but would recommend one set std length and one set long pattern (whatever flavor you prefer....), as when you need two of the same size you are likely to need some leverage.
I don't care for the double open end sets, and only have double box in offsets that don't come any other way.
I like having ratchet wrenches in the sizes I use the most, which for me is SAE full set (3/8 - 7/8), and Metric in 10, 12, and 14. If I were still racing I'd have at least one 15/16...
Other tools I "need" 2 (or more) of:
Clamps, you (I) cannot have too many of any type!
If you fabricate, welders, C grip vise grips
Extensions. All too often I need to get to a specific length, so having more than one in the common lengths, as well as shorty 1" extensions, allows a lot of length adjustment.
I don't tend to have copies of the same ratchet, but I do have different lengths from stubby to extra long, and flex head in a couple of lengths in 3/8. I want then in 1/4" drive and one long one in 1/2" as well, but I'm cheap and they don't come up at yard sales often...
Different lengths of the same blade/tip screwdrivers, particularly #2 Phillips and 1/4" straight blade.
Ditto on the Drills, at least for commonly used sizes; Again if you fabricate a lot...
Power hand drills/screw guns, I like to have one setup for drilling and one for screws, or you can get the quick change bit holders, but then you practicably have to stay within a brand, which can be a pain...
Spare bits in the common sizes for the above they do wear out.
Ditto on jack stands - two pair (at least)
Things you probably DON'T need in duplicate: (leaving aside road boxes and other legit second sources locations)
Nut drivers (if not a full set, get 5/16, 3/8, as these are common for hose clamps...
Flair wrenches
Tappet wrenches
Ignition wrenches (why the combo wrench designers never caught on to the different sizes on each end like they did for ignition wrenches makes me wonder...)
Sockets (other than impact vs. non impact, and deep vs std.)
Specialty tools (pullers, Cherry Picker (handy for more than just engines...), probably an engine stand if you are into major rebuilding,
Torque wrenches other than in larger Lb-Ft and Lb-In...
a set of ramps
Pliers other then different styles.
Floor jack are debatable. One is generally sufficient, but having two (or three) can be very handy...