@@ -13,6 +13,7 @@
 
 ## Fixes
 
+- #553 - fix `An item with the same key has already been added` when a service field resolver builds an object from multiple context members (e.g. `(ctx, srv) => srv.Get(new Thing { A = ctx.A, B = ctx.B })`) and that field is selected below another service field
 - #498 - fix ExecutableDirectiveLocation naming regression introduced in beta3
 
 # 6.0.0-beta3
 
@@ -31,7 +31,9 @@ public ExpressionReplacer(IEnumerable<GraphQLExtractedField> expressionsToReplac
         {
             foreach (var exp in field.FieldExpressions)
             {
-                this.expressionsToReplace.Add(exp, field);
+                // The extractor deliberately emits repeats - the same instance appears twice
+                // when a resolver builds one object from several context members.
+                this.expressionsToReplace[exp] = field;
             }
         }
     }
 
@@ -1376,6 +1376,48 @@ public void TestCollectionWithServiceUsingNewInExpression()
         Assert.Equal("resolvedSettings", Enumerable.ElementAt(people[0].GetType().GetFields(), 0).Name);
     }
 
+    /// <summary>
+    /// A resolver that builds an object from more than one context member, as an argument to the service call,
+    /// has both member reads credited to the enclosing new/init expression - so ExpressionExtractor records that
+    /// single node instance twice. Below another service field the bulk path is unavailable and ExpressionReplacer
+    /// is built from those expressions, which used to throw "An item with the same key has already been added".
+    /// </summary>
+    [Fact]
+    public void TestServiceFieldWithNewInServiceCallArgUnderServiceField()
+    {
+        var schema = SchemaBuilder.FromObject<TestDataContext>();
+        schema.AddType<ProjectConfig>("ProjectConfig").AddAllFields();
+        // outer service field - returns a User
+        schema.UpdateType<Project>(type => type.ReplaceField("createdBy", "Get user that created it").Resolve<UserService>((proj, users) => users.GetUserById(proj.CreatedBy)));
+        // inner service field - builds one object from 2 context members inside the service call
+        schema.Type<User>().AddField("config", "Config for the user").Resolve<ConfigService>((user, srv) => srv.Get(new Project { Id = user.Id, Name = user.Field2 }));
+
+        var gql = new QueryRequest { Query = "{ projects { name createdBy { id config { type } } } }" };
+
+        var context = new TestDataContext
+        {
+            Projects =
+            [
+                new Project
+                {
+                    Id = 1,
+                    CreatedBy = 7,
+                    Name = "Project 1",
+                },
+            ],
+        };
+        var serviceCollection = new ServiceCollection();
+        serviceCollection.AddSingleton(new UserService());
+        serviceCollection.AddSingleton(new ConfigService());
+        var sp = serviceCollection.BuildServiceProvider();
+
+        var res = schema.ExecuteRequestWithContext(gql, context, sp, null);
+        Assert.Null(res.Errors);
+        dynamic projects = res.Data!["projects"]!;
+        Assert.Equal(7, projects[0].createdBy.id);
+        Assert.Equal("Something", projects[0].createdBy.config.type);
+    }
+
     [Fact]
     public void TestRootServiceFieldBackToContext()
     {
 
@@ -50,6 +50,21 @@ public void ExtractLongMemberExpressionSameNameInMethod()
         Assert.Equal(((MethodCallExpression)expression.Body).Arguments[1], extracted.ElementAt(1).Value.First());
     }
 
+    [Fact]
+    public void ExtractObjectBuiltFromMultipleMembersInMethodArg()
+    {
+        // Building an object from 2 context members as an argument to the service call - both member reads are
+        // credited to the enclosing expression, so the same node instance is extracted twice under the one name.
+        // ExpressionReplacer relies on tolerating that repeat - see TestServiceFieldWithNewInServiceCallArgUnderServiceField
+        Expression<Func<User, ConfigService, ProjectConfig>> expression = (user, srv) => srv.Get(new Project { Id = user.Id, Name = user.Field2 });
+        var extractor = new ExpressionExtractor();
+        var extracted = extractor.Extract(expression.Body, expression.Parameters[0], false);
+        Assert.NotNull(extracted);
+        Assert.Single(extracted);
+        var arg = ((MethodCallExpression)expression.Body).Arguments[0];
+        Assert.Equal([arg, arg], extracted.First().Value);
+    }
+
     [Fact]
     public void ExtractExpressionInAsync()
     {
Original file line number	Diff line number	Diff line change
`@@ -31,7 +31,9 @@ public ExpressionReplacer(IEnumerable<GraphQLExtractedField> expressionsToReplac`
`31`	`31`	`{`
`32`	`32`	`foreach (var exp in field.FieldExpressions)`
`33`	`33`	`{`
`34`		`- this.expressionsToReplace.Add(exp, field);`
	`34`	`+ // The extractor deliberately emits repeats - the same instance appears twice`
	`35`	`+ // when a resolver builds one object from several context members.`
	`36`	`+ this.expressionsToReplace[exp] = field;`
`35`	`37`	`}`
`36`	`38`	`}`
`37`	`39`	`}`