diff --git a/parser/python.lalrpop b/parser/python.lalrpop
index a73d835..70ad18f 100644
--- a/parser/python.lalrpop
+++ b/parser/python.lalrpop
@@ -6,7 +6,7 @@
 use crate::{
     ast,
     error::{LexicalError, LexicalErrorType},
-    function::{ArgumentList, parse_args, parse_params},
+    function::{ArgumentList, parse_args, parse_params, validate_arguments},
     lexer,
     context::set_context,
     string::parse_strings,
@@ -552,16 +552,20 @@ FuncDef: ast::Stmt = {
 };
 
 Parameters: ast::Arguments = {
-    "(" <a: (ParameterList<TypedParameter>)?> ")" => {
-        a.unwrap_or_else(|| ast::Arguments {
-            posonlyargs: vec![],
-            args: vec![],
-            vararg: None,
-            kwonlyargs: vec![],
-            kw_defaults: vec![],
-            kwarg: None,
-            defaults: vec![]
-        })
+    "(" <a: (ParameterList<TypedParameter>)?> ")" =>? {
+        let args = validate_arguments(
+            a.unwrap_or_else(|| ast::Arguments {
+                posonlyargs: vec![],
+                args: vec![],
+                vararg: None,
+                kwonlyargs: vec![],
+                kw_defaults: vec![],
+                kwarg: None,
+                defaults: vec![]
+            })
+        )?;
+
+        Ok(args)
     }
 };
 
@@ -774,19 +778,22 @@ NamedExpression: ast::Expr = {
 };
 
 LambdaDef: ast::Expr = {
-    <location:@L> "lambda" <p:ParameterList<UntypedParameter>?> ":" <body:Test<"all">> <end_location:@R> => {
-        let p = p.unwrap_or_else(|| {
-            ast::Arguments {
-                posonlyargs: vec![],
-                args: vec![],
-                vararg: None,
-                kwonlyargs: vec![],
-                kw_defaults: vec![],
-                kwarg: None,
-                defaults: vec![]
+    <location:@L> "lambda" <p:ParameterList<UntypedParameter>?> ":" <body:Test<"all">> <end_location:@R> =>? {
+        let p = validate_arguments(
+            p.unwrap_or_else(|| {
+                ast::Arguments {
+                    posonlyargs: vec![],
+                    args: vec![],
+                    vararg: None,
+                    kwonlyargs: vec![],
+                    kw_defaults: vec![],
+                    kwarg: None,
+                    defaults: vec![]
+                }
             }
-        });
-        ast::Expr {
+        ))?;
+
+        Ok(ast::Expr {
             location,
             end_location: Some(end_location),
             custom: (),
@@ -794,7 +801,7 @@ LambdaDef: ast::Expr = {
                 args: Box::new(p),
                 body: Box::new(body)
             }
-        }
+        })
     }
 }
 
diff --git a/parser/src/error.rs b/parser/src/error.rs
index 47096a5..b9d7cbf 100644
--- a/parser/src/error.rs
+++ b/parser/src/error.rs
@@ -21,6 +21,7 @@ pub enum LexicalErrorType {
     TabError,
     TabsAfterSpaces,
     DefaultArgumentError,
+    DuplicateArgumentError,
     PositionalArgumentError,
     UnpackedArgumentError,
     DuplicateKeywordArgumentError,
@@ -50,6 +51,9 @@ impl fmt::Display for LexicalErrorType {
             LexicalErrorType::DefaultArgumentError => {
                 write!(f, "non-default argument follows default argument")
             }
+            LexicalErrorType::DuplicateArgumentError => {
+                write!(f, "duplicate argument in function definition")
+            }
             LexicalErrorType::DuplicateKeywordArgumentError => {
                 write!(f, "keyword argument repeated")
             }
diff --git a/parser/src/function.rs b/parser/src/function.rs
index 4662113..adbb269 100644
--- a/parser/src/function.rs
+++ b/parser/src/function.rs
@@ -10,6 +10,39 @@ pub struct ArgumentList {
 type ParameterDefs = (Vec<ast::Arg>, Vec<ast::Arg>, Vec<ast::Expr>);
 type ParameterDef = (ast::Arg, Option<ast::Expr>);
 
+pub fn validate_arguments(
+    arguments: ast::Arguments
+) -> Result<ast::Arguments, LexicalError> {
+    let mut all_args: Vec<&ast::Located<ast::ArgData>> = vec![];
+
+    all_args.extend(arguments.posonlyargs.iter());
+    all_args.extend(arguments.args.iter());
+
+    if let Some(a) = &arguments.vararg {
+        all_args.push(a);
+    }
+
+    all_args.extend(arguments.kwonlyargs.iter());
+
+    if let Some(a) = &arguments.kwarg {
+        all_args.push(a);
+    }
+
+    let mut all_arg_names =
+        FxHashSet::with_hasher(Default::default());
+    for arg in all_args {
+        let arg_name = arg.node.arg.clone();
+        if !all_arg_names.insert(arg_name) {
+            return Err(LexicalError {
+                error: LexicalErrorType::DuplicateArgumentError,
+                location: arg.location,
+            });
+        }
+    }
+
+    return Ok(arguments);
+}
+
 pub fn parse_params(
     params: (Vec<ParameterDef>, Vec<ParameterDef>),
 ) -> Result<ParameterDefs, LexicalError> {