diff --git a/crates/engine/src/cmd/styles.rs b/crates/engine/src/cmd/styles.rs
index a5f2b78f..552a44cf 100644
--- a/crates/engine/src/cmd/styles.rs
+++ b/crates/engine/src/cmd/styles.rs
@@ -112,6 +112,9 @@ pub fn specs() -> Vec<CommandSpec> {
         spec!("variable.createCollection", "Create variable collection", "{name, modes? [names]}", true, |s, p| {
             let name = req_string("variable.createCollection", p, "name")?;
             let doc = s.doc_mut()?;
+            if doc.variables.collections.iter().any(|c| c.name == name) {
+                return Err(bad("variable.createCollection", format!("a collection named `{name}` already exists")));
+            }
             let id = doc.new_collection_id();
             let modes: Vec<String> = match get(p, "modes") {
                 Some(Value::Array(a)) => a.iter().filter_map(|m| m.as_str().map(str::to_string)).collect(),
diff --git a/crates/engine/src/variables_editor_tests.rs b/crates/engine/src/variables_editor_tests.rs
index 815fcb2b..9518911e 100644
--- a/crates/engine/src/variables_editor_tests.rs
+++ b/crates/engine/src/variables_editor_tests.rs
@@ -114,3 +114,14 @@ fn field_scopes_by_property_and_layer() {
     assert_eq!(scope(f, "paddingHorizontal"), Some("GAP"));
     assert_eq!(scope(f, "visible"), None);
 }
+
+#[test]
+fn create_collection_refuses_a_taken_name() {
+    let mut s = Session::new();
+    ex(&mut s, "variable.createCollection", json!({"name": "Theme"}));
+    let before = s.doc().unwrap().variables.collections.len();
+    assert!(s.execute("variable.createCollection", &json!({"name": "Theme"})).is_err());
+    assert_eq!(s.doc().unwrap().variables.collections.len(), before);
+    ex(&mut s, "variable.createCollection", json!({"name": "Theme 2"}));
+    assert_eq!(s.doc().unwrap().variables.collections.len(), before + 1);
+}