This switch can be set in OntoConfig.prp via:

EliminateDuplicates = <on|off>

During the evaluation process of a query (sub-) results may be inferred in different ways (e.g. a tuple can be stored in some facts table or it can be inferred by a rule). Duplicates can also occur as the result of join operations. If EliminateDuplicates=on (this is the default) then all duplicates are removed from the result tuples. Removing duplicates might be a costly operation and there are some use cases where you simply do not care about duplicates. In this case it is recommended to set EliminateDuplicates=off as the result is returned faster.