[web] add duplicate citation key detection and warning message in BibTex editor

GitOrigin-RevId: c126f05e0b9a0f7e37b5ff9e06833ba52fb9c34c

services/web/frontend/extracted-translations.json

@@ -191,6 +191,7 @@
   "beta_program_already_participating": "",
   "beta_program_benefits": "",
   "beta_program_not_participating": "",
+  "bibtex_duplicates_warning": "",
   "billed_annually_at": "",
   "billed_monthly_at": "",
   "billed_yearly": "",

services/web/locales/en.json

@@ -246,6 +246,7 @@
   "beta_program_opt_in_action": "Opt-in to beta program",
   "beta_program_opt_out_action": "Opt-out of beta program",
   "bibliographies": "Bibliographies",
+  "bibtex_duplicates_warning": "This file contains duplicate citation keys: __keys__. Only the first occurrence of each duplicate key is shown.",
   "billed_annually_at": "Billed annually at <0>__price__</0> <1>(includes plan and any add-ons)</1>",
   "billed_monthly_at": "Billed monthly at <0>__price__</0> <1>(includes plan and any add-ons)</1>",
   "billed_yearly": "billed yearly",