diff --git a/crates/compiler/can/src/annotation.rs b/crates/compiler/can/src/annotation.rs
index c3fc13e13a..deb85be333 100644
--- a/crates/compiler/can/src/annotation.rs
+++ b/crates/compiler/can/src/annotation.rs
@@ -127,7 +127,7 @@ pub struct NamedVariable {
 pub struct AbleVariable {
     pub variable: Variable,
     pub name: Lowercase,
-    pub abilities: Vec<Symbol>,
+    pub abilities: VecSet<Symbol>,
     // NB: there may be multiple occurrences of a variable
     pub first_seen: Region,
 }
@@ -166,7 +166,7 @@ impl IntroducedVariables {
         self.named.insert(named_variable);
     }
 
-    pub fn insert_able(&mut self, name: Lowercase, var: Loc<Variable>, abilities: Vec<Symbol>) {
+    pub fn insert_able(&mut self, name: Lowercase, var: Loc<Variable>, abilities: VecSet<Symbol>) {
         self.debug_assert_not_already_present(var.value);
 
         let able_variable = AbleVariable {
@@ -539,7 +539,11 @@ fn can_annotation_help(
 
                 // Generate an variable bound to the ability so we can keep compiling.
                 let var = var_store.fresh();
-                introduced_variables.insert_able(fresh_ty_var, Loc::at(region, var), vec![symbol]);
+                introduced_variables.insert_able(
+                    fresh_ty_var,
+                    Loc::at(region, var),
+                    VecSet::singleton(symbol),
+                );
                 return Type::Variable(var);
             }
 
@@ -930,7 +934,7 @@ fn canonicalize_has_clause(
     );
     let var_name = Lowercase::from(var_name);
 
-    let mut can_abilities = Vec::with_capacity(abilities.len());
+    let mut can_abilities = VecSet::with_capacity(abilities.len());
     for &Loc {
         region,
         value: ability,
@@ -957,10 +961,10 @@ fn canonicalize_has_clause(
         };
 
         references.insert(ability);
-        if can_abilities.contains(&ability) {
+        let already_seen = can_abilities.insert(ability);
+
+        if already_seen {
             env.problem(roc_problem::can::Problem::DuplicateHasAbility { ability, region });
-        } else {
-            can_abilities.push(ability);
         }
     }
 
diff --git a/crates/compiler/can/src/module.rs b/crates/compiler/can/src/module.rs
index 24487cea18..0b8e9b6793 100644
--- a/crates/compiler/can/src/module.rs
+++ b/crates/compiler/can/src/module.rs
@@ -136,7 +136,7 @@ pub struct Module {
 #[derive(Debug, Default)]
 pub struct RigidVariables {
     pub named: MutMap<Variable, Lowercase>,
-    pub able: MutMap<Variable, (Lowercase, Vec<Symbol>)>,
+    pub able: MutMap<Variable, (Lowercase, VecSet<Symbol>)>,
     pub wildcards: VecSet<Variable>,
 }
 
diff --git a/crates/compiler/collections/src/vec_set.rs b/crates/compiler/collections/src/vec_set.rs
index d5fd02a63c..492fe7014a 100644
--- a/crates/compiler/collections/src/vec_set.rs
+++ b/crates/compiler/collections/src/vec_set.rs
@@ -1,6 +1,6 @@
 use std::{borrow::Borrow, iter::FromIterator};
 
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
 pub struct VecSet<T> {
     elements: Vec<T>,
 }
@@ -26,6 +26,12 @@ impl<T: PartialEq> VecSet<T> {
         }
     }
 
+    pub fn singleton(element: T) -> Self {
+        Self {
+            elements: vec![element],
+        }
+    }
+
     pub fn len(&self) -> usize {
         self.elements.len()
     }
@@ -145,3 +151,11 @@ impl<T> From<Vec<T>> for VecSet<T> {
         Self { elements }
     }
 }
+
+impl<T> std::ops::Deref for VecSet<T> {
+    type Target = [T];
+
+    fn deref(&self) -> &Self::Target {
+        &self.elements
+    }
+}