diff --git a/frontend/cypress/e2e/project/project-view-kanban.spec.ts b/frontend/cypress/e2e/project/project-view-kanban.spec.ts
index 55e3150db..f338ea075 100644
--- a/frontend/cypress/e2e/project/project-view-kanban.spec.ts
+++ b/frontend/cypress/e2e/project/project-view-kanban.spec.ts
@@ -35,17 +35,17 @@ function createSingleTaskInBucket(count = 1, attrs = {}) {
 }
 
 function createTaskWithBuckets(buckets, count = 1) {
-	const data = TaskFactory.create(10, {
-		project_id: 1,
-	})
-	TaskBucketFactory.truncate()
-	data.forEach(t => TaskBucketFactory.create(count, {
-		task_id: t.id,
-		bucket_id: buckets[0].id,
-		project_view_id: buckets[0].project_view_id,
-	}, false))
+       const data = TaskFactory.create(count, {
+               project_id: 1,
+       })
+       TaskBucketFactory.truncate()
+       data.forEach(t => TaskBucketFactory.create(1, {
+               task_id: t.id,
+               bucket_id: buckets[0].id,
+               project_view_id: buckets[0].project_view_id,
+       }, false))
 
-	return data
+       return data
 }
 
 describe('Project View Kanban', () => {
diff --git a/pkg/migration/20250624092830.go b/pkg/migration/20250624092830.go
new file mode 100644
index 000000000..e17384e29
--- /dev/null
+++ b/pkg/migration/20250624092830.go
@@ -0,0 +1,44 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package migration
+
+import (
+	"src.techknowlogick.com/xormigrate"
+	"xorm.io/xorm"
+	"xorm.io/xorm/schemas"
+)
+
+func init() {
+	migrations = append(migrations, &xormigrate.Migration{
+		ID:          "20250624092830",
+		Description: "add unique index for task buckets",
+		Migrate: func(tx *xorm.Engine) error {
+			var query string
+			switch tx.Dialect().URI().DBType {
+			case schemas.MYSQL:
+				query = "CREATE UNIQUE INDEX UQE_task_buckets_task_project_view ON task_buckets (task_id, project_view_id)"
+			default:
+				query = "CREATE UNIQUE INDEX IF NOT EXISTS UQE_task_buckets_task_project_view ON task_buckets (task_id, project_view_id)"
+			}
+			_, err := tx.Exec(query)
+			return err
+		},
+		Rollback: func(tx *xorm.Engine) error {
+			return nil
+		},
+	})
+}
diff --git a/pkg/models/kanban_task_bucket.go b/pkg/models/kanban_task_bucket.go
index e8f49574a..0da154993 100644
--- a/pkg/models/kanban_task_bucket.go
+++ b/pkg/models/kanban_task_bucket.go
@@ -25,13 +25,20 @@ import (
 	"xorm.io/xorm"
 )
 
+// TaskBucket represents the relation between a task and a kanban bucket.
+// A task can only appear once per project view which is ensured by a
+// unique index on the combination of task_id and project_view_id.
 type TaskBucket struct {
-	BucketID      int64   `xorm:"bigint not null index" json:"bucket_id" param:"bucket"`
-	Bucket        *Bucket `xorm:"-" json:"bucket"`
-	TaskID        int64   `xorm:"bigint not null index" json:"task_id"`
-	ProjectViewID int64   `xorm:"bigint not null index" json:"project_view_id" param:"view"`
-	ProjectID     int64   `xorm:"-" json:"-" param:"project"`
-	Task          *Task   `xorm:"-" json:"task"`
+	BucketID int64   `xorm:"bigint not null index" json:"bucket_id" param:"bucket"`
+	Bucket   *Bucket `xorm:"-" json:"bucket"`
+	// The task which belongs to the bucket. Together with ProjectViewID
+	// this field is part of a unique index to prevent duplicates.
+	TaskID int64 `xorm:"bigint not null index unique(task_view)" json:"task_id"`
+	// The view this bucket belongs to. Combined with TaskID this forms a
+	// unique index.
+	ProjectViewID int64 `xorm:"bigint not null index unique(task_view)" json:"project_view_id" param:"view"`
+	ProjectID     int64 `xorm:"-" json:"-" param:"project"`
+	Task          *Task `xorm:"-" json:"task"`
 
 	web.Rights   `xorm:"-" json:"-"`
 	web.CRUDable `xorm:"-" json:"-"`