In the anion ${\mathrm{HCOO}}^{-}$ the two carbon-oxygen bonds are found to be of equal length. The reason for this fact is that

the $\mathrm{C}=\mathrm{O}$ bond is weaker that the $\mathrm{C}-\mathrm{O}$ bond

the anion is obtained by removal of a proton from the acid molecule

the anion has two resonating structures

the electronic orbitals of carbon atom are hybridised