diff --git a/cozo-core/src/runtime/imperative.rs b/cozo-core/src/runtime/imperative.rs
index a2ce96f5..fb9a15ff 100644
--- a/cozo-core/src/runtime/imperative.rs
+++ b/cozo-core/src/runtime/imperative.rs
@@ -333,28 +333,34 @@ impl SessionTx<'_> {
         rels: &NamedRows,
         cur_vld: ValidityTs,
     ) -> Result<()> {
+        let mut key_bindings = vec![];
+        for k in rels.headers.iter() {
+            let k = k.replace('(', "_").replace(')', "");
+            let k = Symbol::new(k.clone(), Default::default());
+            if key_bindings.contains(&k) {
+                bail!("Duplicate variable name {}, please use distinct variables in `as` construct.", k);
+            }
+            key_bindings.push(k);
+        }
+        let keys = key_bindings
+            .iter()
+            .map(|s| ColumnDef {
+                name: s.name.clone(),
+                typing: NullableColType {
+                    coltype: ColType::Any,
+                    nullable: true,
+                },
+                default_gen: None,
+            })
+            .collect_vec();
+
         let meta = InputRelationHandle {
             name: Symbol::new(name, Default::default()),
             metadata: StoredRelationMetadata {
-                keys: rels
-                    .headers
-                    .iter()
-                    .map(|s| ColumnDef {
-                        name: s.into(),
-                        typing: NullableColType {
-                            coltype: ColType::Any,
-                            nullable: true,
-                        },
-                        default_gen: None,
-                    })
-                    .collect_vec(),
+                keys,
                 non_keys: vec![],
             },
-            key_bindings: rels
-                .headers
-                .iter()
-                .map(|s| Symbol::new(s.clone(), Default::default()))
-                .collect_vec(),
+            key_bindings,
             dep_bindings: vec![],
             span: Default::default(),
         };
diff --git a/cozo-core/src/runtime/tests.rs b/cozo-core/src/runtime/tests.rs
index 3365b269..049867c2 100644
--- a/cozo-core/src/runtime/tests.rs
+++ b/cozo-core/src/runtime/tests.rs
@@ -1322,7 +1322,9 @@ fn as_store_in_imperative_script() {
         )
         .unwrap();
     assert_eq!(res.into_json()["rows"], json!([[1, 2, 3], [4, 5, 6]]));
-    let res = db.run_script(r#"
+    let res = db
+        .run_script(
+            r#"
     {
         ?[y] <- [[1], [2], [3]]
         :create a {x default rand_uuid_v1() => y}
@@ -1331,9 +1333,42 @@ fn as_store_in_imperative_script() {
     {
         ?[x] := *_last{_kind: 'inserted', x}
     }
-    "#, Default::default()).unwrap();
+    "#,
+            Default::default(),
+        )
+        .unwrap();
     assert_eq!(3, res.rows.len());
     for row in res.into_json()["rows"].as_array().unwrap() {
         println!("{}", row);
     }
+    assert!(db
+        .run_script(
+            r#"
+    {
+        ?[x, x] := x = 1
+    } as _last
+    "#,
+            Default::default()
+        )
+        .is_err());
+
+    let res = db
+        .run_script(
+            r#"
+    {
+        x[y] <- [[1], [2], [3]]
+        ?[sum(y)] := x[y]
+    } as _last
+    {
+        ?[sum_y] := *_last{sum_y}
+    }
+    "#,
+            Default::default(),
+        )
+        .unwrap();
+    assert_eq!(1, res.rows.len());
+    for row in res.into_json()["rows"].as_array().unwrap() {
+        println!("{}", row);
+    }
+
 }