fail on duplicate key in map declaration

src/value/map.rs

@@ -56,14 +56,16 @@ impl SassMap {
             .collect()
     }
 
-    pub fn insert(&mut self, key: Value, value: Value) {
-        for &mut (ref k, ref mut v) in &mut self.0 {
+    /// Returns true if the key already exists
+    pub fn insert(&mut self, key: Value, value: Value) -> bool {
+        for (ref k, ref mut v) in &mut self.0 {
             if k == &key {
                 *v = value;
-                return;
+                return true;
             }
         }
         self.0.push((key, value));
+        false
     }
 }
 

src/value/parse.rs

@@ -325,7 +325,9 @@ impl Value {
                             super_selector,
                         )?;
                         devour_whitespace(paren_toks);
-                        map.insert(key, val);
+                        if map.insert(key, val) {
+                            return Err("Duplicate key.".into());
+                        }
                         if paren_toks.peek().is_none() {
                             break;
                         }

tests/map.rs

@@ -129,3 +129,7 @@ test!(
     "a {\n  color: inspect(map-remove((\"foo\": 1, \"bar\": 2, \"baz\": 3), \"bar\", \"baz\"));\n}\n",
     "a {\n  color: (\"foo\": 1);\n}\n"
 );
+error!(
+    duplicate_key_in_declaration,
+    "a {\n  $a: (foo: a, foo: b);;\n}\n", "Error: Duplicate key."
+);