diff --git a/crates/formality-check/src/adts.rs b/crates/formality-check/src/adts.rs
index dde84ab1..dcf8df85 100644
--- a/crates/formality-check/src/adts.rs
+++ b/crates/formality-check/src/adts.rs
@@ -1,3 +1,6 @@
+use std::collections::HashSet;
+
+use anyhow::bail;
 use formality_prove::Env;
 use formality_rust::grammar::{Adt, AdtBoundData, Field, Variant};
 use formality_types::grammar::Fallible;
@@ -6,6 +9,20 @@ impl super::Check<'_> {
     pub(super) fn check_adt(&self, adt: &Adt) -> Fallible<()> {
         let Adt { id: _, binder } = adt;
 
+        // names is used to check that there are no name conflicts
+        let mut names = HashSet::new();
+        for Variant { name, fields } in &adt.binder.peek().variants {
+            if !names.insert((name, None)) {
+                bail!("variant \"{name:?}\" defined multiple times");
+            }
+            let vname = name;
+            for Field { name, ty: _ } in fields {
+                if !names.insert((vname, Some(name))) {
+                    bail!("field \"{name:?}\" of variant \"{vname:?}\" defined multiple times");
+                }
+            }
+        }
+
         let mut env = Env::default();
 
         let AdtBoundData {
@@ -15,8 +32,6 @@ impl super::Check<'_> {
 
         self.prove_where_clauses_well_formed(&env, &where_clauses, &where_clauses)?;
 
-        // FIXME: check names are unique or integers from 0..n
-
         for Variant { name: _, fields } in &variants {
             for Field { name: _, ty } in fields {
                 self.prove_goal(&env, &where_clauses, ty.well_formed())?;
diff --git a/src/test/mod.rs b/src/test/mod.rs
index 7a66c1be..0e5b3870 100644
--- a/src/test/mod.rs
+++ b/src/test/mod.rs
@@ -135,6 +135,42 @@ fn basic_where_clauses_fail() {
     )
 }
 
+#[test]
+fn basic_adt_variant_dup() {
+    crate::assert_err!(
+        [
+            crate Foo {
+                enum Bar {
+                    Baz{},
+                    Baz{},
+                }
+            }
+        ]
+
+        [ r#"variant "Baz" defined multiple times"#, ]
+
+        expect_test::expect![[r#"variant "Baz" defined multiple times"#]]
+    )
+}
+
+#[test]
+fn basic_adt_field_dup() {
+    crate::assert_err!(
+        [
+            crate Foo {
+                struct Bar {
+                    baz: (),
+                    baz: (),
+                }
+            }
+        ]
+
+        [ r#"field "baz" of variant "struct" defined multiple times"#, ]
+
+        expect_test::expect![[r#"field "baz" of variant "struct" defined multiple times"#]]
+    )
+}
+
 #[test]
 fn trait_items_with_duplicate_fn_names() {
     crate::assert_err!(