If you go to the Model Generator tab on the Solver|Options command's dialog box you will see a checkbox titled Check for duplicates names in data and model . When this option is enabled, LINGO will compare primal set member names to all the variable names used in the model's equations. If any duplicates are found, LINGO will print the first three and print this error message. To enable this option in command-line versions use the SET CHKDUP 1 command.