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

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;
+    }
 }
 
 /**