fix: only apply prefix once (#255)

Simple check that we do not add a configured prefix more than once, as
was happening in certain situations

Author: runeb

packages/gatsby-source-sanity/src/util/normalize.ts

@@ -82,6 +82,10 @@ export function getTypeName(type: string, typePrefix: string | undefined) {
     return type
   }
 
+  if (typePrefix && type.startsWith(typePrefix)) {
+    return type
+  }
+
   const typeName = startCase(type)
   if (scalarTypeNames.includes(typeName)) {
     return typeName