Add check for obviously different TypeReferences (#896)

codegen/type.go

@@ -1,15 +1,29 @@
 package codegen
 
 import (
+	"fmt"
+
 	"github.com/99designs/gqlgen/codegen/config"
 )
 
 func (b *builder) buildTypes() map[string]*config.TypeReference {
 	ret := map[string]*config.TypeReference{}
+	var key string
+	var existing *config.TypeReference
+	var found bool
 
 	for _, ref := range b.Binder.References {
 		for ref != nil {
-			ret[ref.UniquenessKey()] = ref
+			key = ref.UniquenessKey()
+			if existing, found = ret[key]; found {
+				// Simplistic check of content which is obviously different.
+				existingGQL := fmt.Sprintf("%v", existing.GQL)
+				newGQL := fmt.Sprintf("%v", ref.GQL)
+				if existingGQL != newGQL {
+					panic(fmt.Sprintf("non-unique key \"%s\", trying to replace %s with %s", key, existingGQL, newGQL))
+				}
+			}
+			ret[key] = ref
 
 			ref = ref.Elem()
 		}