Solution : https://service.sap.com/sap/support/notes/40319 (SAP Service marketplace login required)
Key words : 
additional key words pcon_300 pcon_30a pcon_30b pcon_30c pcon_30d pcon_30e pcon_30f pcon31gupgrade1024database object exists, database object exists, upgrade phase pcon_30c, phase pcon_<rel>, symptom sql error 1024, database object, sql statement, upgrade hotline, database level, table vbdata
Related Notes : 
       
| 315377 | Additional information about upgrading to 3.1I SR1 | 
| 91594 | Additional Info: Upgrading to Release 3.1I | 
| 76604 | Additional Info: Upgrading to 3.1H | 
| 64570 | Additional Info: Upgrading to 3.1G | 
| 48632 | Additional Info: Upgrading to 3.0F | 
| 43288 | Additional Info: Upgrading to 3.0E | 
| 37387 | Additional Info: Upgrading to 3.0D |