added test for duplicate keys

since only implement Eq and not Ord for Glob, inspects all pairs.
Fails on first duplicate found as wouldn't expect to be adding large
number of syntaxes at once and emits the glob string pattern duplicated.

src/syntax_mapping.rs

@@ -286,4 +286,20 @@ mod tests {
             Some(MappingTarget::MapToUnknown)
         );
     }
+
+    #[test]
+    /// verifies that SyntaxMapping::builtin() doesn't repeat `Glob`-based keys
+    fn no_duplicate_builtin_keys() {
+        let mappings = SyntaxMapping::builtin().mappings;
+        for i in 0..mappings.len() {
+            let mut tail = mappings[i + 1..].into_iter();
+            match tail.find(|item| item.0.glob() == mappings[i].0.glob()) {
+                Some(duplicate) => panic!(
+                    "duplicate in SyntaxMapping::builtin() for glob {}",
+                    duplicate.0.glob().glob()
+                ),
+                None => (),
+            }
+        }
+    }
 }