[schoen]

Apparently, although it's not likely to occur under most conditions, you can form a monatomic carbon C²⁺ ion by shooting a laser into very high-temperature carbon:

https://sciencetrends.com/the-charge-of-a-carbon-ion/

https://aip.scitation.org/doi/10.1063/1.4966987

"Up to C⁴⁺ ions are observed."

And I guess you could also write the C²⁺ ion as "C⁺⁺" if you wanted. :-)

[Dylan16807]

That just makes the comparison worse, if it's difficult and unstable and almost immediately sheds the +s.