template: provide more useful error message on duplicate keys in template

Updated the error message thrown when duplicate mapping keys are encountered during template rendering to provide additional context on how to resolve the duplicate key issue.

Author: garrettdieckmann

lib/github_provider.js

@@ -68,7 +68,7 @@ class GitHubContentsApi {
                 }
             }))
             .catch(e => this._handleResponseError(e, `get contents for ${contentPath}`))
-            .then(resp => JSON.parse(resp.data).content)
+            .then(resp => resp.data.content)
             .then(data => Buffer.from(data, 'base64').toString('utf8'));
     }
 }

lib/template.js

@@ -201,7 +201,7 @@ function JsonPostProcessStrategy(rendered) {
     if (rendered.trim() === '') {
         rendered = '""';
     }
-    return JSON.stringify(yaml.load(rendered) || '', null, 2);
+    return JSON.stringify(LoadYamlWithDuplicateKeyCheck(rendered) || '', null, 2);
 }
 
 /**
@@ -211,7 +211,32 @@ function YamlPostProcessStrategy(rendered) {
     if (rendered.trim() === '') {
         rendered = '""';
     }
-    return yaml.dump(yaml.load(rendered));
+    return yaml.dump(LoadYamlWithDuplicateKeyCheck(rendered));
+}
+
+/**
+ * LoadYamlWithDuplicateKeyCheck will load a yaml string
+ * and throw a custom error response if duplicate mapping keys exist
+ */
+function LoadYamlWithDuplicateKeyCheck(str) {
+    const duplicateKeyRegex = /"(.*?)"/g;
+    try {
+        return yaml.load(str);
+    } catch (e) {
+        if (e.message.match(/^duplicated mapping key/)) {
+            const seenKeys = new Set();
+            let match;
+            // eslint-disable-next-line no-cond-assign
+            while ((match = duplicateKeyRegex.exec(e.message)) !== null) {
+                const matchContents = match[1];
+                if (seenKeys.has(matchContents)) {
+                    throw new Error(`parameters must have unique values. The parameter value "${matchContents}" is duplicated across parameters and you must update the parameter values to ensure they are unique.`);
+                }
+                seenKeys.add(matchContents);
+            }
+        }
+        throw e;
+    }
 }
 
 /**