diff --git a/README.md b/README.md
index 8b079051..69d32d40 100644
--- a/README.md
+++ b/README.md
@@ -216,7 +216,7 @@ The derive implementation supports the following attributes:
 - `codec(encoded_as = "OtherType")`: Needs to be placed above a field and makes the field being
   encoded by using `OtherType`.
 - `codec(index = 0)`: Needs to be placed above an enum variant to make the variant use the given
-  index when encoded. By default the index is determined by counting from `0` beginning wth the
+  index when encoded. By default the index is determined by counting from `0` beginning with the
   first variant.
 - `codec(encode_bound)`, `codec(decode_bound)` and `codec(mel_bound)`: All 3 attributes take
   in a `where` clause for the `Encode`, `Decode` and `MaxEncodedLen` trait implementation for
diff --git a/derive/src/decode.rs b/derive/src/decode.rs
index 1e228776..593305c2 100644
--- a/derive/src/decode.rs
+++ b/derive/src/decode.rs
@@ -44,10 +44,18 @@ pub fn quote(
 				Ok(variants) => variants,
 				Err(e) => return e.to_compile_error(),
 			};
-
-			let recurse = variants.iter().enumerate().map(|(i, v)| {
+			match utils::check_indexes(variants.iter()).map_err(|e| e.to_compile_error()) {
+				Ok(()) => (),
+				Err(e) => return e,
+			};
+			let mut items = vec![];
+			for (index, v) in variants.iter().enumerate() {
 				let name = &v.ident;
-				let index = utils::variant_index(v, i);
+				let index = match utils::variant_index(v, index).map_err(|e| e.into_compile_error())
+				{
+					Ok(i) => i,
+					Err(e) => return e,
+				};
 
 				let create = create_instance(
 					quote! { #type_name #type_generics :: #name },
@@ -57,7 +65,7 @@ pub fn quote(
 					crate_path,
 				);
 
-				quote_spanned! { v.span() =>
+				let item = quote_spanned! { v.span() =>
 					#[allow(clippy::unnecessary_cast)]
 					__codec_x_edqy if __codec_x_edqy == #index as ::core::primitive::u8 => {
 						// NOTE: This lambda is necessary to work around an upstream bug
@@ -68,8 +76,9 @@ pub fn quote(
 							#create
 						})();
 					},
-				}
-			});
+				};
+				items.push(item);
+			}
 
 			let read_byte_err_msg =
 				format!("Could not decode `{type_name}`, failed to read variant byte");
@@ -79,7 +88,7 @@ pub fn quote(
 				match #input.read_byte()
 					.map_err(|e| e.chain(#read_byte_err_msg))?
 				{
-					#( #recurse )*
+					#( #items )*
 					_ => {
 						#[allow(clippy::redundant_closure_call)]
 						return (move || {
diff --git a/derive/src/encode.rs b/derive/src/encode.rs
index df7af38a..4df20a7c 100644
--- a/derive/src/encode.rs
+++ b/derive/src/encode.rs
@@ -306,12 +306,19 @@ fn impl_encode(data: &Data, type_name: &Ident, crate_path: &syn::Path) -> TokenS
 			if variants.is_empty() {
 				return quote!();
 			}
-
-			let recurse = variants.iter().enumerate().map(|(i, f)| {
+			match utils::check_indexes(variants.iter()).map_err(|e| e.to_compile_error()) {
+				Ok(()) => (),
+				Err(e) => return e,
+			};
+			let mut items = vec![];
+			for (index, f) in variants.iter().enumerate() {
 				let name = &f.ident;
-				let index = utils::variant_index(f, i);
-
-				match f.fields {
+				let index = match utils::variant_index(f, index).map_err(|e| e.into_compile_error())
+				{
+					Ok(i) => i,
+					Err(e) => return e,
+				};
+				let item = match f.fields {
 					Fields::Named(ref fields) => {
 						let fields = &fields.named;
 						let field_name = |_, ident: &Option<Ident>| quote!(#ident);
@@ -389,12 +396,12 @@ fn impl_encode(data: &Data, type_name: &Ident, crate_path: &syn::Path) -> TokenS
 
 						[hinting, encoding]
 					},
-				}
-			});
-
-			let recurse_hinting = recurse.clone().map(|[hinting, _]| hinting);
-			let recurse_encoding = recurse.clone().map(|[_, encoding]| encoding);
+				};
+				items.push(item)
+			}
 
+			let recurse_hinting = items.iter().map(|[hinting, _]| hinting);
+			let recurse_encoding = items.iter().map(|[_, encoding]| encoding);
 			let hinting = quote! {
 				// The variant index uses 1 byte.
 				1_usize + match *#self_ {
diff --git a/derive/src/lib.rs b/derive/src/lib.rs
index 2f9ea7d9..8ba6d3de 100644
--- a/derive/src/lib.rs
+++ b/derive/src/lib.rs
@@ -94,8 +94,8 @@ fn wrap_with_dummy_const(
 /// * if variant has attribute: `#[codec(index = "$n")]` then n
 /// * else if variant has discriminant (like 3 in `enum T { A = 3 }`) then the discriminant.
 /// * else its position in the variant set, excluding skipped variants, but including variant with
-/// discriminant or attribute. Warning this position does collision with discriminant or attribute
-/// index.
+///   discriminant or attribute. Warning this position does collision with discriminant or attribute
+///   index.
 ///
 /// variant attributes:
 /// * `#[codec(skip)]`: the variant is not encoded.
diff --git a/derive/src/utils.rs b/derive/src/utils.rs
index 735e2348..cd3d5375 100644
--- a/derive/src/utils.rs
+++ b/derive/src/utils.rs
@@ -17,9 +17,9 @@
 //! NOTE: attributes finder must be checked using check_attribute first,
 //! otherwise the macro can panic.
 
-use std::str::FromStr;
+use std::{collections::HashMap, str::FromStr};
 
-use proc_macro2::TokenStream;
+use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use syn::{
 	parse::Parse, punctuated::Punctuated, spanned::Spanned, token, Attribute, Data, DataEnum,
@@ -38,11 +38,29 @@ where
 	})
 }
 
+/// check usage of variant indexes with #[scale(index = $int)] attribute or
+/// explicit  discriminant on the variant
+pub fn check_indexes<'a, I: Iterator<Item = &'a &'a Variant>>(values: I) -> syn::Result<()> {
+	let mut map: HashMap<u8, Span> = HashMap::new();
+	for (i, v) in values.enumerate() {
+		let index = variant_index(v, i)?;
+		if let Some(span) = map.insert(index, v.span()) {
+			let mut error = syn::Error::new(
+				v.span(),
+				"scale codec error: Invalid variant index, the variant index is duplicated.",
+			);
+			error.combine(syn::Error::new(span, "Variant index used here."));
+			return Err(error);
+		}
+	}
+	Ok(())
+}
+
 /// Look for a `#[scale(index = $int)]` attribute on a variant. If no attribute
 /// is found, fall back to the discriminant or just the variant index.
-pub fn variant_index(v: &Variant, i: usize) -> TokenStream {
+pub fn variant_index(v: &Variant, index: usize) -> syn::Result<u8> {
 	// first look for an attribute
-	let index = find_meta_item(v.attrs.iter(), |meta| {
+	let codec_index = find_meta_item(v.attrs.iter(), |meta| {
 		if let Meta::NameValue(ref nv) = meta {
 			if nv.path.is_ident("index") {
 				if let Expr::Lit(ExprLit { lit: Lit::Int(ref v), .. }) = nv.value {
@@ -56,14 +74,30 @@ pub fn variant_index(v: &Variant, i: usize) -> TokenStream {
 
 		None
 	});
-
-	// then fallback to discriminant or just index
-	index.map(|i| quote! { #i }).unwrap_or_else(|| {
-		v.discriminant
-			.as_ref()
-			.map(|(_, expr)| quote! { #expr })
-			.unwrap_or_else(|| quote! { #i })
-	})
+	if let Some(index) = codec_index {
+		Ok(index)
+	} else {
+		match v.discriminant.as_ref() {
+			Some((_, syn::Expr::Lit(ExprLit { lit: syn::Lit::Int(v), .. }))) => {
+				let byte = v.base10_parse::<u8>().expect(
+					"scale codec error: Invalid variant index, discriminant doesn't fit u8.",
+				);
+				Ok(byte)
+			},
+			Some((_, expr)) => Err(syn::Error::new(
+				expr.span(),
+				"scale codec error: Invalid discriminant, only int literal are accepted, e.g. \
+					`= 32`.",
+			)),
+			None => index.try_into().map_err(|_| {
+				syn::Error::new(
+					v.span(),
+					"scale codec error: Variant index is too large, only 256 variants are \
+					supported.",
+				)
+			}),
+		}
+	}
 }
 
 /// Look for a `#[codec(encoded_as = "SomeType")]` outer attribute on the given
diff --git a/tests/scale_codec_ui/codec_duplicate_index.rs b/tests/scale_codec_ui/codec_duplicate_index.rs
new file mode 100644
index 00000000..dc3e666e
--- /dev/null
+++ b/tests/scale_codec_ui/codec_duplicate_index.rs
@@ -0,0 +1,17 @@
+#[derive(::parity_scale_codec::Decode, ::parity_scale_codec::Encode)]
+#[codec(crate = ::parity_scale_codec)]
+enum T {
+	A = 3,
+	#[codec(index = 3)]
+	B,
+}
+
+#[derive(::parity_scale_codec::Decode, ::parity_scale_codec::Encode)]
+#[codec(crate = ::parity_scale_codec)]
+enum T1 {
+	A,
+	#[codec(index = 0)]
+	B,
+}
+
+fn main() {}
diff --git a/tests/scale_codec_ui/codec_duplicate_index.stderr b/tests/scale_codec_ui/codec_duplicate_index.stderr
new file mode 100644
index 00000000..afa68f85
--- /dev/null
+++ b/tests/scale_codec_ui/codec_duplicate_index.stderr
@@ -0,0 +1,23 @@
+error: scale codec error: Invalid variant index, the variant index is duplicated.
+ --> tests/scale_codec_ui/codec_duplicate_index.rs:5:2
+  |
+5 |     #[codec(index = 3)]
+  |     ^
+
+error: Variant index used here.
+ --> tests/scale_codec_ui/codec_duplicate_index.rs:4:2
+  |
+4 |     A = 3,
+  |     ^
+
+error: scale codec error: Invalid variant index, the variant index is duplicated.
+  --> tests/scale_codec_ui/codec_duplicate_index.rs:13:2
+   |
+13 |     #[codec(index = 0)]
+   |     ^
+
+error: Variant index used here.
+  --> tests/scale_codec_ui/codec_duplicate_index.rs:12:2
+   |
+12 |     A,
+   |     ^
diff --git a/tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs b/tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs
new file mode 100644
index 00000000..03836635
--- /dev/null
+++ b/tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs
@@ -0,0 +1,16 @@
+#[derive(::parity_scale_codec::Decode, ::parity_scale_codec::Encode)]
+#[codec(crate = ::parity_scale_codec)]
+enum T {
+	A = 1,
+	B,
+}
+
+#[derive(::parity_scale_codec::Decode, ::parity_scale_codec::Encode)]
+#[codec(crate = ::parity_scale_codec)]
+enum T2 {
+	#[codec(index = 1)]
+	A,
+	B,
+}
+
+fn main() {}
diff --git a/tests/scale_codec_ui/discriminant_variant_counted_in_default_index.stderr b/tests/scale_codec_ui/discriminant_variant_counted_in_default_index.stderr
new file mode 100644
index 00000000..8ca151b3
--- /dev/null
+++ b/tests/scale_codec_ui/discriminant_variant_counted_in_default_index.stderr
@@ -0,0 +1,23 @@
+error: scale codec error: Invalid variant index, the variant index is duplicated.
+ --> tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs:5:2
+  |
+5 |     B,
+  |     ^
+
+error: Variant index used here.
+ --> tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs:4:2
+  |
+4 |     A = 1,
+  |     ^
+
+error: scale codec error: Invalid variant index, the variant index is duplicated.
+  --> tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs:13:2
+   |
+13 |     B,
+   |     ^
+
+error: Variant index used here.
+  --> tests/scale_codec_ui/discriminant_variant_counted_in_default_index.rs:11:2
+   |
+11 |     #[codec(index = 1)]
+   |     ^
diff --git a/tests/variant_number.rs b/tests/variant_number.rs
index 9bdaba0a..0c71689c 100644
--- a/tests/variant_number.rs
+++ b/tests/variant_number.rs
@@ -1,18 +1,6 @@
 use parity_scale_codec::Encode;
 use parity_scale_codec_derive::Encode as DeriveEncode;
 
-#[test]
-fn discriminant_variant_counted_in_default_index() {
-	#[derive(DeriveEncode)]
-	enum T {
-		A = 1,
-		B,
-	}
-
-	assert_eq!(T::A.encode(), vec![1]);
-	assert_eq!(T::B.encode(), vec![1]);
-}
-
 #[test]
 fn skipped_variant_not_counted_in_default_index() {
 	#[derive(DeriveEncode)]
@@ -27,14 +15,16 @@ fn skipped_variant_not_counted_in_default_index() {
 }
 
 #[test]
-fn index_attr_variant_counted_and_reused_in_default_index() {
+fn index_attr_variant_duplicates_indices() {
+	// Tests codec index overriding and that variant indexes are without duplicates
 	#[derive(DeriveEncode)]
 	enum T {
+		#[codec(index = 0)]
+		A = 1,
 		#[codec(index = 1)]
-		A,
-		B,
+		B = 0,
 	}
 
-	assert_eq!(T::A.encode(), vec![1]);
+	assert_eq!(T::A.encode(), vec![0]);
 	assert_eq!(T::B.encode(), vec![1]);
 }