Avoid duplicates in fetch expressions

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XML

CRM

I have the entities “collection” and “store” associated with a N:N relationship “collection_store”. I want to use a fetchXML to retrieve the collections associated to a store, by avoiding duplicates (I’m expecting to retrieve only two records even if one collection is associated to two different stores): to achieve it, inside the fetch expression I changed the value of the property “distinct” to “true” but it did not work.

What I have tried:

static List<Entity> GetCollectionsInStore(CrmServiceClient serviceClient)
{
     FetchExpression query = new FetchExpression(@"<fetch version=""1.0"" output-format= ""xml-platform"" mapping =""logical"" distinct=""true"">
	<entity name=""collection"">
           <attribute name=""collectionid"" />
           <link-entity name=""collection_store"" from=""collectionid"" to=""collectionid"" intersect=""true"">
             <all-attributes />
           </link-entity>
        </entity>
</fetch> ");

var collectionsInStore = new List<Entity>();
collectionsInStore = serviceClient.RetrieveMultiple(query).Entities.ToList();
return collectionsInStore ;
}