Distinct values selection in list column

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SharePoint2013

Here i am selecting top 500 rows from the list using below rest api method, I want to select only distinct values for (ESI_ProjectSeqNoLookupId ) column .Please explain in below code where to insert distinct keyword or someother way to select column in ESI_ProjectSeqNoLookupId.

query.RestQuery = "getbytitle('RFA')/Items?$select=ID,ESI_AFENo,ESI_ProjectSeqNoLookupId&$top=500";

What I have tried:

where to use distinct keyword i dont know