Add duplicate keyword argument error

Fixes RustPython#116

Author: vazrupe

Cargo.lock

@@ -21,6 +21,7 @@ pub enum LexicalErrorType {
     UnicodeError,
     NestingError,
     PositionalArgumentError,
+    DuplicateKeywordArgumentError,
     UnrecognizedToken { tok: char },
     FStringError(FStringErrorType),
     OtherError(String),
@@ -33,6 +34,9 @@ impl fmt::Display for LexicalErrorType {
             LexicalErrorType::FStringError(error) => write!(f, "Got error in f-string: {}", error),
             LexicalErrorType::UnicodeError => write!(f, "Got unexpected unicode"),
             LexicalErrorType::NestingError => write!(f, "Got unexpected nesting"),
+            LexicalErrorType::DuplicateKeywordArgumentError => {
+                write!(f, "keyword argument repeated")
+            }
             LexicalErrorType::PositionalArgumentError => {
                 write!(f, "positional argument follows keyword argument")
             }

parser/src/error.rs

@@ -1,3 +1,5 @@
+use std::collections::HashSet;
+
 use crate::ast;
 use crate::error::{LexicalError, LexicalErrorType};
 
@@ -6,9 +8,22 @@ type FunctionArgument = (Option<Option<String>>, ast::Expression);
 pub fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ast::ArgumentList, LexicalError> {
     let mut args = vec![];
     let mut keywords = vec![];
+
+    let mut keyword_names = HashSet::with_capacity(func_args.len());
     for (name, value) in func_args {
         match name {
             Some(n) => {
+                if let Some(keyword_name) = n.clone() {
+                    if keyword_names.contains(&keyword_name) {
+                        return Err(LexicalError {
+                            error: LexicalErrorType::DuplicateKeywordArgumentError,
+                            location: value.location.clone(),
+                        });
+                    }
+
+                    keyword_names.insert(keyword_name.clone());
+                }
+
                 keywords.push(ast::Keyword { name: n, value });
             }
             None => {

parser/src/function.rs

@@ -95,3 +95,10 @@ def func(**kwargs):
 
 kwargs = func(a=1, b=2, c=3)
 assert kwargs == [('a', 1), ('b', 2), ('c', 3)]
+
+
+def inc(n):
+    return n + 1
+
+with assert_raises(SyntaxError):
+    exec("inc(n=1, n=2)")