diff --git a/src/model/trip.rs b/src/model/trip.rs
index da9e0bf..9df8756 100644
--- a/src/model/trip.rs
+++ b/src/model/trip.rs
@@ -65,6 +65,7 @@ FROM user_trip WHERE trip_details_id = (SELECT trip_details_id FROM trip WHERE i
         .unwrap() //TODO: fixme
     }
 
+    /// Cox decides to create own trip.
     pub async fn new_own(db: &SqlitePool, cox_id: i64, trip_details_id: i64) {
         sqlx::query!(
             "INSERT INTO trip (cox_id, trip_details_id) VALUES(?, ?)",
@@ -76,17 +77,39 @@ FROM user_trip WHERE trip_details_id = (SELECT trip_details_id FROM trip WHERE i
         .unwrap(); //TODO: fixme
     }
 
-    /// Returns true if successfully inserted; false if not (e.g. because user is already
-    /// participant
-    pub async fn new_join(db: &SqlitePool, cox_id: i64, planned_event_id: i64) -> bool {
-        sqlx::query!(
+    /// Cox decides to help in a planned event.
+    pub async fn new_join(
+        db: &SqlitePool,
+        cox_id: i64,
+        planned_event_id: i64,
+    ) -> Result<(), CoxHelpError> {
+        let is_rower = sqlx::query!(
+            "SELECT count(*) as amount
+            FROM user_trip
+            WHERE trip_details_id =
+                (SELECT trip_details_id FROM planned_event WHERE id = ?)
+            AND user_id = ?",
+            planned_event_id,
+            cox_id
+        )
+        .fetch_one(db)
+        .await
+        .unwrap();
+        if is_rower.amount > 0 {
+            return Err(CoxHelpError::AlreadyRegisteredAsRower);
+        }
+
+        match sqlx::query!(
             "INSERT INTO trip (cox_id, planned_event_id) VALUES(?, ?)",
             cox_id,
             planned_event_id
         )
         .execute(db)
         .await
-        .is_ok()
+        {
+            Ok(_) => Ok(()),
+            Err(_) => Err(CoxHelpError::AlreadyRegisteredAsCox),
+        }
     }
 
     pub async fn delete(db: &SqlitePool, user_id: i64, planned_event_id: i64) {
@@ -99,11 +122,9 @@ FROM user_trip WHERE trip_details_id = (SELECT trip_details_id FROM trip WHERE i
         .await
         .is_ok();
     }
-
-    //pub async fn delete(db: &SqlitePool, id: i64) {
-    //    sqlx::query!("DELETE FROM planned_event WHERE id = ?", id)
-    //        .execute(db)
-    //        .await
-    //        .unwrap(); //TODO: fixme
-    //}
+}
+
+pub enum CoxHelpError {
+    AlreadyRegisteredAsRower,
+    AlreadyRegisteredAsCox,
 }
diff --git a/src/rest/cox.rs b/src/rest/cox.rs
index 8054cd6..4e5d206 100644
--- a/src/rest/cox.rs
+++ b/src/rest/cox.rs
@@ -6,7 +6,11 @@ use rocket::{
 };
 use sqlx::SqlitePool;
 
-use crate::model::{trip::Trip, tripdetails::TripDetails, user::CoxUser};
+use crate::model::{
+    trip::{CoxHelpError, Trip},
+    tripdetails::TripDetails,
+    user::CoxUser,
+};
 
 //TODO: add constraints (e.g. planned_amount_cox > 0)
 #[derive(FromForm)]
@@ -37,10 +41,15 @@ async fn create(db: &State<SqlitePool>, data: Form<AddTripForm>, cox: CoxUser) -
 
 #[get("/join/<planned_event_id>")]
 async fn join(db: &State<SqlitePool>, planned_event_id: i64, cox: CoxUser) -> Flash<Redirect> {
-    if Trip::new_join(db, cox.id, planned_event_id).await {
-        Flash::success(Redirect::to("/"), "Danke für's helfen!")
-    } else {
-        Flash::error(Redirect::to("/"), "Du nimmst bereits teil!")
+    match Trip::new_join(db, cox.id, planned_event_id).await {
+        Ok(_) => Flash::success(Redirect::to("/"), "Danke für's helfen!"),
+        Err(CoxHelpError::AlreadyRegisteredAsCox) => {
+            Flash::error(Redirect::to("/"), "Du hilfst bereits aus!")
+        }
+        Err(CoxHelpError::AlreadyRegisteredAsRower) => Flash::error(
+            Redirect::to("/"),
+            "Du hast dich bereits als Ruderer angemeldet!",
+        ),
     }
 }