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node_modules/graphql/validation/rules/UniqueArgumentNames.js.flow

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+// @flow strict
+
+import { GraphQLError } from '../../error/GraphQLError';
+import { type ASTVisitor } from '../../language/visitor';
+
+import { type ASTValidationContext } from '../ValidationContext';
+
+export function duplicateArgMessage(argName: string): string {
+  return `There can be only one argument named "${argName}".`;
+}
+
+/**
+ * Unique argument names
+ *
+ * A GraphQL field or directive is only valid if all supplied arguments are
+ * uniquely named.
+ */
+export function UniqueArgumentNames(context: ASTValidationContext): ASTVisitor {
+  let knownArgNames = Object.create(null);
+  return {
+    Field() {
+      knownArgNames = Object.create(null);
+    },
+    Directive() {
+      knownArgNames = Object.create(null);
+    },
+    Argument(node) {
+      const argName = node.name.value;
+      if (knownArgNames[argName]) {
+        context.reportError(
+          new GraphQLError(duplicateArgMessage(argName), [
+            knownArgNames[argName],
+            node.name,
+          ]),
+        );
+      } else {
+        knownArgNames[argName] = node.name;
+      }
+      return false;
+    },
+  };
+}