Add check for unique redirect URLs

gatsby-node.js

@@ -28,6 +28,9 @@ exports.modifyWebpackConfig = ({config, stage}) => {
 exports.createPages = async ({graphql, boundActionCreators}) => {
   const {createPage, createRedirect} = boundActionCreators;
 
+  // Used to detect and prevent duplicate redirects
+  const redirectToSlugMap = {};
+
   const blogTemplate = resolve('./src/templates/blog.js');
   const communityTemplate = resolve('./src/templates/community.js');
   const docsTemplate = resolve('./src/templates/docs.js');
@@ -114,13 +117,22 @@ exports.createPages = async ({graphql, boundActionCreators}) => {
           redirect = [redirect];
         }
 
-        redirect.forEach(fromPath =>
+        redirect.forEach(fromPath => {
+          if (redirectToSlugMap[fromPath] != null) {
+            console.error(`Duplicate redirect detected from "${fromPath}" to:\n` +
+              `* ${redirectToSlugMap[fromPath]}\n` +
+              `* ${slug}\n`
+            );
+            process.exit(1);
+          }
+
+          redirectToSlugMap[fromPath] = slug;
           createRedirect({
             fromPath: `/${fromPath}`,
             redirectInBrowser: true,
             toPath: `/${slug}`,
-          }),
+          });
-        );
+        });
       }
     }
   });