How do I stop artifacts from showing up twice in Report Builder when using Lifecycle Query Engine, when showing the Artifact Type?

They are not duplicates as such. Requirements often have multiple RDF types and then the results include each of those types in the results.When reporting on artifacts whose columns have multiple values, you get the Cartesian product. This is how queries work, regardless of SPARQL, SQL, Cypher etc.

If you report on Requirement (an OSLC defined resource), the Type property represents the name of the resource shape associated with each requirement. If you report on a specific subtype of Requirement ( e.g. Feature), the Type property represents the RDF types of those requirements, not the name of the instance shape. Requirements commonly have multiple RDF types. However, in such a use case, the requirements will all be of associated with resource shapes of the same name and that name is already known. For example, when reporting on Features, the shape name is always "Feature".