This reaction is often considered an unwanted side-reaction but it is also investigated as a synthetic tool.
[1] An early pioneer was Christoph Grundmann reporting on a carbene dimerisation in 1938.
For example, ethyl diazoacetate is converted to diethyl maleate using the ruthenium catalyst chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium:[3] Grubbs' catalyst is also effective [4] In this reaction type the active intermediate is a transition metal carbene complex.
A diazo cross-coupling reaction has also been reported between ethyl diazoacetate and methyl phenyldiazoacetate using the rhodium catalyst [Rh2(OPiv)4].
[5] A direct metal carbene dimerization has been used in the synthesis of novel Polyalkynylethenes [6]