How do electrons repel each other in multiple bonds?

A typical $C - C$ bond is $1.54 \times {10}^{-} 10 \cdot m$ long; a $C = C$ bond is $1.45 \times {10}^{-} 10 \cdot m$ long; a $C \equiv C$ bond is $1.38 \times {10}^{-} 10 \cdot m$ long.
The more electrons between the carbon nuclei, the closer they can approach each other WITHOUT internuclear repulsion. And the average bond enthalpies reflect this interaction; certainly this is observed for $C \equiv C$, ""^(-)C-=O^+, and $N \equiv N$ bonds.