diff --git a/src/justfile.rs b/src/justfile.rs
index 501134821e..ca112b77aa 100644
--- a/src/justfile.rs
+++ b/src/justfile.rs
@@ -1,3 +1,4 @@
+use parallel::Ran;
 use std::sync::Arc;
 use {super::*, serde::Serialize};
 
@@ -254,14 +255,10 @@ impl<'src> Justfile<'src> {
       search,
     };
 
-    // let mut ran = BTreeSet::new();
+    let ran = Ran::new();
     parallel::task_scope(config.parallel, |scope| {
       for (recipe, arguments) in grouped {
-        scope.run(|| {
-          Self::run_recipe(
-            &context, recipe, arguments, &dotenv, search, /*&mut ran*/
-          )
-        })?;
+        scope.run(|| Self::run_recipe(&context, recipe, arguments, &dotenv, search, &ran))?;
       }
       Ok(())
     })?;
@@ -287,16 +284,16 @@ impl<'src> Justfile<'src> {
     arguments: &[&str],
     dotenv: &BTreeMap<String, String>,
     search: &Search,
-    // ran: &mut BTreeSet<Vec<String>>,
+    ran: &Ran,
   ) -> RunResult<'src, ()> {
     let mut invocation = vec![recipe.name().to_owned()];
     for argument in arguments {
       invocation.push((*argument).to_string());
     }
 
-    // if ran.contains(&invocation) {
-    //   return Ok(());
-    // }
+    if ran.contains(&invocation) {
+      return Ok(());
+    }
 
     let (outer, positional) = Evaluator::evaluate_parameters(
       context.config,
@@ -327,7 +324,7 @@ impl<'src> Justfile<'src> {
             &arguments.iter().map(String::as_ref).collect::<Vec<&str>>(),
             dotenv,
             search,
-            // ran,
+            ran,
           )
         })?;
       }
@@ -337,7 +334,7 @@ impl<'src> Justfile<'src> {
     recipe.run(context, dotenv, scope.child(), search, &positional)?;
 
     {
-      // let mut ran = BTreeSet::new();
+      let ran = Ran::new();
 
       parallel::task_scope(context.config.parallel, |scope| {
         for Dependency { recipe, arguments } in recipe.dependencies.iter().skip(recipe.priors) {
@@ -351,22 +348,25 @@ impl<'src> Justfile<'src> {
             );
           }
 
-          scope.run(move || {
-            Self::run_recipe(
-              context,
-              recipe,
-              &evaluated.iter().map(String::as_ref).collect::<Vec<&str>>(),
-              dotenv,
-              search,
-              // &mut ran,
-            )
+          scope.run({
+            let ran = ran.clone();
+            move || {
+              Self::run_recipe(
+                context,
+                recipe,
+                &evaluated.iter().map(String::as_ref).collect::<Vec<&str>>(),
+                dotenv,
+                search,
+                &ran,
+              )
+            }
           })?;
         }
         Ok(())
       })?;
     }
 
-    // ran.insert(invocation);
+    ran.insert(invocation);
     Ok(())
   }
 
diff --git a/src/parallel.rs b/src/parallel.rs
index d38990e8c7..a30e91d518 100644
--- a/src/parallel.rs
+++ b/src/parallel.rs
@@ -1,5 +1,7 @@
 use crate::RunResult;
 use crossbeam::thread;
+use std::collections::HashSet;
+use std::sync::{Arc, Mutex};
 
 type ScopeResult<'src> = RunResult<'src, ()>;
 
@@ -10,6 +12,7 @@ pub(crate) struct TaskScope<'env, 'src, 'inner_scope> {
 }
 
 impl<'env, 'src, 'inner_scope> TaskScope<'env, 'src, 'inner_scope> {
+  /// run the given task, either directly synchronously or spawned in a background thread.
   pub(crate) fn run<'scope, F>(&'scope mut self, f: F) -> ScopeResult<'src>
   where
     'src: 'env,
@@ -31,6 +34,9 @@ impl<'env, 'src, 'inner_scope> TaskScope<'env, 'src, 'inner_scope> {
 /// run. The first error will be returned as result of this `task_scope`.
 ///
 /// Only works for tasks with an `RunResult<'src, ()>` result type.
+///
+/// When `parallel` is set to `false`, the tasks are directly executed
+/// when calling `run`.
 pub(crate) fn task_scope<'env, 'src, F>(parallel: bool, f: F) -> ScopeResult<'src>
 where
   F: for<'inner_scope> FnOnce(&mut TaskScope<'env, 'src, 'inner_scope>) -> ScopeResult<'src>,
@@ -51,3 +57,41 @@ where
   })
   .expect("could not join thread")
 }
+
+/// track which tasks were already run, across all running threads.
+#[derive(Clone)]
+pub(crate) struct Ran(Arc<Mutex<HashSet<Vec<String>>>>);
+
+impl Ran {
+  pub(crate) fn new() -> Self {
+    Self(Arc::new(Mutex::new(HashSet::new())))
+  }
+
+  pub(crate) fn insert(&self, args: Vec<String>) {
+    let mut ran = self.0.lock().unwrap();
+    ran.insert(args);
+  }
+
+  pub(crate) fn contains(&self, args: &Vec<String>) -> bool {
+    let ran = self.0.lock().unwrap();
+    ran.contains(args)
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn test_ran_empty() {
+    let r = Ran::new();
+    assert!(!r.contains(&vec![]));
+  }
+
+  #[test]
+  fn test_ran_insert_contains() {
+    let r = Ran::new();
+    r.insert(vec!["1".into(), "2".into(), "3".into()]);
+    assert!(r.contains(&vec!["1".into(), "2".into(), "3".into()]));
+  }
+}