diff --git a/packages/mrml-core/src/prelude/parser/mod.rs b/packages/mrml-core/src/prelude/parser/mod.rs
index c0e538ad..9f368d55 100644
--- a/packages/mrml-core/src/prelude/parser/mod.rs
+++ b/packages/mrml-core/src/prelude/parser/mod.rs
@@ -1,6 +1,7 @@
 use std::marker::PhantomData;
 
 use htmlparser::{StrSpan, Tokenizer};
+use indexmap::map::Entry;
 
 use self::loader::IncludeLoaderError;
 use super::hash::Map;
@@ -394,10 +395,14 @@ pub(crate) fn parse_attributes_map(
 ) -> Result<Map<String, Option<String>>, Error> {
     let mut result = Map::new();
     while let Some(attr) = cursor.next_attribute()? {
-        result.insert(
-            attr.qualified_name(),
-            attr.value.map(|inner| inner.to_string()),
-        );
+        match result.entry(attr.qualified_name()) {
+            Entry::Vacant(slot) => {
+                slot.insert(attr.value.map(|inner| inner.to_string()));
+            }
+            Entry::Occupied(_) => {
+                cursor.add_warning(WarningKind::DuplicateAttribute, attr.span);
+            }
+        }
     }
     Ok(result)
 }
@@ -630,3 +635,32 @@ macro_rules! should_not_async_parse {
         });
     };
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{MrmlCursor, MrmlParser, ParserOptions, WarningKind};
+    use crate::mj_text::MjText;
+
+    #[test]
+    fn should_warn_and_keep_first_value_on_duplicate_attribute() {
+        let raw = r#"<mj-text font-size="12px" font-size="20px">hi</mj-text>"#;
+        let opts = ParserOptions::default();
+        let parser = MrmlParser::new(&opts);
+        let mut cursor = MrmlCursor::new(raw);
+        let element: MjText = parser.parse_root(&mut cursor).unwrap();
+
+        assert_eq!(
+            element
+                .attributes
+                .get("font-size")
+                .and_then(|v| v.as_deref()),
+            Some("12px")
+        );
+
+        let warnings = cursor.warnings();
+        assert_eq!(warnings.len(), 1);
+        assert_eq!(warnings[0].kind, WarningKind::DuplicateAttribute);
+        // The span should point at the second occurrence, not the first.
+        assert_eq!(warnings[0].span.start, raw.rfind("font-size").unwrap());
+    }
+}
diff --git a/packages/mrml-core/src/prelude/parser/output.rs b/packages/mrml-core/src/prelude/parser/output.rs
index 00f9cada..d9f35e49 100644
--- a/packages/mrml-core/src/prelude/parser/output.rs
+++ b/packages/mrml-core/src/prelude/parser/output.rs
@@ -6,6 +6,7 @@ pub struct ParseOutput<E> {
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum WarningKind {
     UnexpectedAttribute,
+    DuplicateAttribute,
     InlineStyleUnsupported,
 }
 
@@ -13,6 +14,7 @@ impl WarningKind {
     pub const fn as_str(&self) -> &'static str {
         match self {
             Self::UnexpectedAttribute => "unexpected-attribute",
+            Self::DuplicateAttribute => "duplicate-attribute",
             Self::InlineStyleUnsupported => "inline-style-unsupported",
         }
     }
@@ -22,6 +24,7 @@ impl std::fmt::Display for WarningKind {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Self::UnexpectedAttribute => f.write_str("unexpected attribute"),
+            Self::DuplicateAttribute => f.write_str("duplicate attribute"),
             Self::InlineStyleUnsupported => {
                 f.write_str("inlining styles is not supported in this build")
             }
diff --git a/packages/mrml-wasm/src/parser/mod.rs b/packages/mrml-wasm/src/parser/mod.rs
index 46e43679..835d1d31 100644
--- a/packages/mrml-wasm/src/parser/mod.rs
+++ b/packages/mrml-wasm/src/parser/mod.rs
@@ -225,6 +225,7 @@ impl From<mrml::prelude::parser::Span> for Span {
 #[tsify(into_wasm_abi)]
 pub enum WarningKind {
     UnexpectedAttributes,
+    DuplicateAttribute,
     InlineStyleUnsupported,
 }
 
@@ -232,6 +233,7 @@ impl From<mrml::prelude::parser::WarningKind> for WarningKind {
     fn from(value: mrml::prelude::parser::WarningKind) -> Self {
         match value {
             mrml::prelude::parser::WarningKind::UnexpectedAttribute => Self::UnexpectedAttributes,
+            mrml::prelude::parser::WarningKind::DuplicateAttribute => Self::DuplicateAttribute,
             mrml::prelude::parser::WarningKind::InlineStyleUnsupported => {
                 Self::InlineStyleUnsupported
             }