fix(tasks): prevent duplicate task_positions rows and stale identifiers

A task could end up with more than one task_positions row for the same
(task_id, project_view_id): rapid/concurrent creation raced the
check-then-insert paths, and the create path could insert a position that
a triggered RecalculateTaskPositions had already persisted for the new
task. The table had no unique constraint, so the duplicates were stored
silently (#2844).

In the table view this made the LEFT JOIN on task_positions emit the task
twice; getTasksForProjects enriched only the map entry, so the duplicate
slice row kept an empty identifier and rendered as "#N" instead of
"PREFIX-N" (#2725).

- Add a unique index on task_positions(task_id, project_view_id) via a
  dedup migration (mirrors the task_buckets fix in 20250624092830) plus the
  unique(task_view) struct tag so fresh installs get it too.
- Harden the create path: only queue a position insert when one does not
  already exist for the task+view, and dedupe within the batch.
- Dedupe the task slice returned by getTasksForProjects by id, returning
  the enriched entry, so duplicate position rows can never surface a task
  twice or with a missing identifier.

Fixes #2844
Fixes #2725

pkg/migration/20260617153629.go

@@ -0,0 +1,120 @@
+// 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"
+)
+
+type taskPosition20260617153629 struct {
+	TaskID        int64   `xorm:"bigint not null index"`
+	ProjectViewID int64   `xorm:"bigint not null index"`
+	Position      float64 `xorm:"double not null"`
+}
+
+func (taskPosition20260617153629) TableName() string {
+	return "task_positions"
+}
+
+func init() {
+	migrations = append(migrations, &xormigrate.Migration{
+		ID:          "20260617153629",
+		Description: "deduplicate task positions and add a unique index on task_id + project_view_id",
+		Migrate: func(tx *xorm.Engine) error {
+
+			s := tx.NewSession()
+			defer s.Close()
+
+			err := s.Begin()
+			if err != nil {
+				return err
+			}
+
+			// First remove all duplicate entries. A task may only ever have a
+			// single position per view; rapid task creation could race and
+			// insert more than one row before this constraint existed.
+			duplicates := []taskPosition20260617153629{}
+			err = s.
+				Select("task_id, project_view_id").
+				GroupBy("task_id, project_view_id").
+				Having("count(*) > 1").
+				Find(&duplicates)
+			if err != nil {
+				_ = s.Rollback()
+				return err
+			}
+
+			// Keep the lowest position of each group so the result is
+			// deterministic across databases.
+			kept := []taskPosition20260617153629{}
+			for _, dup := range duplicates {
+				row := taskPosition20260617153629{}
+				has, err := s.
+					Where("task_id = ? AND project_view_id = ?", dup.TaskID, dup.ProjectViewID).
+					OrderBy("position ASC").
+					Get(&row)
+				if err != nil {
+					_ = s.Rollback()
+					return err
+				}
+				if !has {
+					continue
+				}
+				kept = append(kept, row)
+			}
+
+			for _, dup := range duplicates {
+				_, err = s.
+					Where("task_id = ? AND project_view_id = ?", dup.TaskID, dup.ProjectViewID).
+					Delete(&taskPosition20260617153629{})
+				if err != nil {
+					_ = s.Rollback()
+					return err
+				}
+			}
+
+			for _, position := range kept {
+				_, err = s.Insert(&position)
+				if err != nil {
+					_ = s.Rollback()
+					return err
+				}
+			}
+
+			err = s.Commit()
+			if err != nil {
+				return err
+			}
+
+			// Then create the unique index
+			var query string
+			switch tx.Dialect().URI().DBType {
+			case schemas.MYSQL:
+				query = "CREATE UNIQUE INDEX UQE_task_positions_task_project_view ON task_positions (task_id, project_view_id)"
+			default:
+				query = "CREATE UNIQUE INDEX IF NOT EXISTS UQE_task_positions_task_project_view ON task_positions (task_id, project_view_id)"
+			}
+			_, err = tx.Exec(query)
+			return err
+		},
+		Rollback: func(_ *xorm.Engine) error {
+			return nil
+		},
+	})
+}

pkg/models/kanban.go

@@ -257,7 +257,7 @@ func GetTasksInBucketsForView(s *xorm.Session, view *ProjectView, projects []*Pr
 			}
 		}
 
-		ts, _, total, err := getRawTasksForProjects(s, projects, auth, opts)
+		ts, total, err := getRawTasksForProjects(s, projects, auth, opts)
 		if err != nil {
 			return nil, err
 		}

pkg/models/project.go

@@ -1370,7 +1370,7 @@ func (p *Project) Delete(s *xorm.Session, a web.Auth) (err error) {
 
 	// Delete all tasks on that project
 	// Using the loop to make sure all related entities to all tasks are properly deleted as well.
-	tasks, _, _, err := getRawTasksForProjects(s, []*Project{p}, a, &taskSearchOptions{})
+	tasks, _, err := getRawTasksForProjects(s, []*Project{p}, a, &taskSearchOptions{})
 	if err != nil {
 		return
 	}

pkg/models/saved_filter_positions_test.go

@@ -79,8 +79,17 @@ func TestCronInsertsNonZeroPosition(t *testing.T) {
 	require.NoError(t, err)
 	require.True(t, exists)
 
+	// Force the task to a zero position in this view to simulate the unhealed
+	// state. A task only ever has one position row per view, so update it if it
+	// already exists (e.g. created with the filter) instead of inserting a duplicate.
 	tp := &TaskPosition{TaskID: task.ID, ProjectViewID: view.ID, Position: 0}
+	hasPosition, err := s.Where("task_id = ? AND project_view_id = ?", task.ID, view.ID).Exist(&TaskPosition{})
+	require.NoError(t, err)
+	if hasPosition {
+		_, err = s.Where("task_id = ? AND project_view_id = ?", task.ID, view.ID).Cols("position").Update(tp)
+	} else {
 		_, err = s.Insert(tp)
+	}
 	require.NoError(t, err)
 
 	_, err = calculateNewPositionForTask(s, u, task, view)

pkg/models/task_collection_test.go

@@ -1898,7 +1898,7 @@ func TestTaskSearchWithExpandSubtasks(t *testing.T) {
 		expand: []TaskCollectionExpandable{TaskCollectionExpandSubtasks},
 	}
 
-	tasks, _, _, err := getRawTasksForProjects(s, []*Project{project}, &user.User{ID: 15}, opts)
+	tasks, _, err := getRawTasksForProjects(s, []*Project{project}, &user.User{ID: 15}, opts)
 	require.NoError(t, err)
 	require.NotEmpty(t, tasks)
 }

pkg/models/task_position.go

@@ -33,9 +33,9 @@ const MinPositionSpacing = 0.01
 
 type TaskPosition struct {
 	// The ID of the task this position is for
-	TaskID int64 `xorm:"bigint not null index" json:"task_id" param:"task" readOnly:"true" doc:"The numeric id of the task this position belongs to. Taken from the URL; ignored in the request body."`
+	TaskID int64 `xorm:"bigint not null index unique(task_view)" json:"task_id" param:"task" readOnly:"true" doc:"The numeric id of the task this position belongs to. Taken from the URL; ignored in the request body."`
 	// The project view this task is related to
-	ProjectViewID int64 `xorm:"bigint not null index" json:"project_view_id" doc:"The id of the project view this position applies to. Positions are stored per view, so the same task has an independent position in each of its project's views."`
+	ProjectViewID int64 `xorm:"bigint not null index unique(task_view)" json:"project_view_id" doc:"The id of the project view this position applies to. Positions are stored per view, so the same task has an independent position in each of its project's views."`
 	// The position of the task - any task project can be sorted as usual by this parameter.
 	// When accessing tasks via kanban buckets, this is primarily used to sort them based on a range
 	// We're using a float64 here to make it possible to put any task within any two other tasks (by changing the number).
@@ -341,6 +341,40 @@ func calculateNewPositionForTask(s *xorm.Session, a web.Auth, t *Task, view *Pro
 	}, nil
 }
 
+type taskPositionKey struct {
+	taskID int64
+	viewID int64
+}
+
+// filterNewTaskPositions returns the positions whose (task_id, project_view_id)
+// row does not exist yet, also deduplicating within the slice. Position creation
+// during task creation can trigger a full recalculation (calculateNewPositionForTask
+// or moveTaskToDoneBuckets) that already persists rows for the new task, so inserting
+// the queued positions unconditionally would violate the unique index on
+// (task_id, project_view_id).
+func filterNewTaskPositions(s *xorm.Session, positions []*TaskPosition) ([]*TaskPosition, error) {
+	filtered := make([]*TaskPosition, 0, len(positions))
+	seen := make(map[taskPositionKey]bool, len(positions))
+	for _, p := range positions {
+		key := taskPositionKey{taskID: p.TaskID, viewID: p.ProjectViewID}
+		if seen[key] {
+			continue
+		}
+		seen[key] = true
+
+		exists, err := s.
+			Where("task_id = ? AND project_view_id = ?", p.TaskID, p.ProjectViewID).
+			Exist(&TaskPosition{})
+		if err != nil {
+			return nil, err
+		}
+		if !exists {
+			filtered = append(filtered, p)
+		}
+	}
+	return filtered, nil
+}
+
 // DeleteOrphanedTaskPositions removes task position records that reference
 // tasks or project views that no longer exist.
 // If dryRun is true, it counts the orphaned records without deleting them.

pkg/models/task_search_bench_test.go

@@ -139,7 +139,7 @@ func BenchmarkTaskSearch(b *testing.B) {
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		s := db.NewSession()
-		resultSlice, _, _, err := getRawTasksForProjects(s, projects, auth, opts)
+		resultSlice, _, err := getRawTasksForProjects(s, projects, auth, opts)
 		if len(resultSlice) == 0 {
 			b.Fatalf("no results found for needle %q", needle)
 		}

pkg/models/tasks.go

@@ -288,11 +288,11 @@ func getTaskIndexFromSearchString(s string) (index int64) {
 	return
 }
 
-func getRawTasksForProjects(s *xorm.Session, projects []*Project, a web.Auth, opts *taskSearchOptions) (tasks []*Task, resultCount int, totalItems int64, err error) {
+func getRawTasksForProjects(s *xorm.Session, projects []*Project, a web.Auth, opts *taskSearchOptions) (tasks []*Task, totalItems int64, err error) {
 
 	// If the user does not have any projects, don't try to get any tasks
 	if len(projects) == 0 {
-		return nil, 0, 0, nil
+		return nil, 0, nil
 	}
 
 	// Get all project IDs and get the tasks
@@ -324,17 +324,18 @@ func getRawTasksForProjects(s *xorm.Session, projects []*Project, a web.Auth, op
 	}
 	tasks, totalItems, err = dbSearcher.Search(opts)
 
-	return tasks, len(tasks), totalItems, err
+	return tasks, totalItems, err
 }
 
 func getTasksForProjects(s *xorm.Session, projects []*Project, a web.Auth, opts *taskSearchOptions, view *ProjectView) (tasks []*Task, resultCount int, totalItems int64, err error) {
-	tasks, resultCount, totalItems, err = getRawTasksForProjects(s, projects, a, opts)
+	tasks, totalItems, err = getRawTasksForProjects(s, projects, a, opts)
 	if err != nil {
 		return nil, 0, 0, err
 	}
 
-	taskMap := make(map[int64]*Task, len(tasks))
-	for _, t := range tasks {
+	rawTasks := tasks
+	taskMap := make(map[int64]*Task, len(rawTasks))
+	for _, t := range rawTasks {
 		taskMap[t.ID] = t
 	}
 
@@ -343,7 +344,22 @@ func getTasksForProjects(s *xorm.Session, projects []*Project, a web.Auth, opts
 		return nil, 0, 0, err
 	}
 
-	return tasks, resultCount, totalItems, err
+	// A task can appear more than once in the raw result when it has duplicate
+	// task_positions rows for the view (the LEFT JOIN multiplies it). Return one
+	// entry per task, in the original sort order, referencing the enriched map
+	// value so its identifier and other data are set. totalItems already counts
+	// distinct tasks, so this also aligns the page size with it.
+	tasks = make([]*Task, 0, len(taskMap))
+	seen := make(map[int64]bool, len(taskMap))
+	for _, t := range rawTasks {
+		if seen[t.ID] {
+			continue
+		}
+		seen[t.ID] = true
+		tasks = append(tasks, taskMap[t.ID])
+	}
+
+	return tasks, len(tasks), totalItems, err
 }
 
 // GetTaskByIDSimple returns a raw task without extra data by the task ID
@@ -984,6 +1000,13 @@ func createTask(s *xorm.Session, t *Task, a web.Auth, updateAssignees bool, setB
 		return err
 	}
 
+	if len(positions) > 0 {
+		positions, err = filterNewTaskPositions(s, positions)
+		if err != nil {
+			return err
+		}
+	}
+
 	if len(positions) > 0 {
 		_, err = s.Insert(&positions)
 		if err != nil {