Double expression (DE) is a World Health Organization-recognized adverse prognostic factor in diffuse large B-cell lymphoma (DLBCL). However, the prognostic value of DE in the genetic subtyping era and potential mechanisms remain to be explored. We enrolled 246 DLBCL patients diagnosed between December 2021 and September 2023 in a Jiangsu Province Hospital cohort and included 930 DLBCL patients from three published studies in an external cohort. Double-expression DLBCL (DEL) in the external cohort was mainly distributed in the OTHER subtype (42.0%), EZB subtype (28.3%), and MCD subtype (15.0%), whereas the MCD subtype exhibited the highest ratio of DEL. DEL was significantly related to unfavorable overall survival (OS) and progression-free survival (PFS) in DLBCL, but only in EZB and OTHER subtypes that DEL retained remarkably adverse impacts on survivals compared to non-DEL. We explored the prognostic value of clinical and genetic parameters in DEL patients and found only ST2 showed better OS than A53 in DEL patients, whereas the other subtypes showed no significant difference. DEL showed similarities with the MCD subtype in mutation profiles. Furthermore, RNA-sequencing analyses exhibited upregulation in tumor proliferation-related pathways in DEL patients, but downregulation in extracellular matrix organization, T-cell activation and proliferation, type II interferon production, and pathways associated with cell death might contribute to the poor outcomes of DEL patients.