His case marked the first time a Bucks County judge, acting independently of a jury, had imposed a death sentence since Pennsylvania reinstated capital punishment in 1978.
In his appeal, filed in September, Diamond argued that in determining his penalty, County Court Judge Rea B. Boylan did not properly weigh his afflictions, which included schizophrenia and a history of delusional and psychotic behavior.
Justice Max Baer wrote that the court has consistently left to the sentencing judge or jury the power to assess the importance of mitigating or aggravating factors. Baer's opinion was signed by five other justices. Justice Thomas G. Saylor wrote a concurring opinion.
Diamond's public defender could not be reached for comment Monday.
Diamond admitted killing Angel Guadalupe, 46, and Reginald Woodson, 52, saying it was to avenge harassment while he was employed at the warehouse.
He is one of five Bucks County defendants on death row and one of 189 inmates statewide sentenced to death. No one has been executed in Pennsylvania since 1999.