fix(migration): reuse existing labels on re-import

Seed the dedup map at the start of insertFromStructure with the importing
user's existing labels, keyed by title + normalized hex color. Previously
the map was empty on each run, so importing the same CSV (or any other
migration format) twice would create a second copy of every label.

Scoped to the user's own labels so imports don't silently link to other
users' labels visible via shared projects.

Fixes #2742

pkg/modules/migration/create_from_structure.go

@@ -27,6 +27,7 @@ import (
 	"code.vikunja.io/api/pkg/models"
 	"code.vikunja.io/api/pkg/modules/background/handler"
 	"code.vikunja.io/api/pkg/user"
+	"code.vikunja.io/api/pkg/utils"
 )
 
 // InsertFromStructure takes a fully nested Vikunja data structure and a user and then creates everything for this user
@@ -57,7 +58,17 @@ func insertFromStructure(s *xorm.Session, str []*models.ProjectWithTasksAndBucke
 
 	log.Debugf("[creating structure] Creating %d projects", len(str))
 
+	// Seed the dedup map with the user's existing labels so re-imports
+	// reuse them instead of creating duplicates (see issue #2742).
 	labels := make(map[string]*models.Label)
+	existingLabels := []*models.Label{}
+	if err = s.Where("created_by_id = ?", user.ID).Find(&existingLabels); err != nil {
+		return err
+	}
+	for _, l := range existingLabels {
+		labels[l.Title+utils.NormalizeHex(l.HexColor)] = l
+	}
+
 	archivedProjects := []int64{}
 
 	childRelations := make(map[int64][]int64)          // old id is the key, slice of old children ids
@@ -436,14 +447,15 @@ func createProjectWithEverything(s *xorm.Session, project *models.ProjectWithTas
 			if label == nil {
 				continue
 			}
-			lb, exists = labels[label.Title+label.HexColor]
+			key := label.Title + utils.NormalizeHex(label.HexColor)
+			lb, exists = labels[key]
 			if !exists {
 				err = label.Create(s, user)
 				if err != nil {
 					return err
 				}
 				log.Debugf("[creating structure] Created new label %d", label.ID)
-				labels[label.Title+label.HexColor] = label
+				labels[key] = label
 				lb = label
 			}
 

pkg/modules/migration/create_from_structure_test.go

@@ -155,4 +155,59 @@ func TestInsertFromStructure(t *testing.T) {
 		assert.NotEqual(t, 0, testStructure[1].Tasks[0].BucketID) // Should get the default bucket
 		assert.NotEqual(t, 0, testStructure[1].Tasks[6].BucketID) // Should get the default bucket
 	})
+	t.Run("reuses existing labels across imports", func(t *testing.T) {
+		db.LoadAndAssertFixtures(t)
+
+		makeStructure := func() []*models.ProjectWithTasksAndBuckets {
+			return []*models.ProjectWithTasksAndBuckets{
+				{
+					Project: models.Project{Title: "Import project"},
+					Tasks: []*models.TaskWithComments{
+						{
+							Task: models.Task{
+								Title: "Task with label",
+								Labels: []*models.Label{
+									{Title: "Mealie", HexColor: "abcdef"},
+								},
+							},
+						},
+					},
+				},
+			}
+		}
+
+		require.NoError(t, InsertFromStructure(makeStructure(), u))
+		require.NoError(t, InsertFromStructure(makeStructure(), u))
+
+		s := db.NewSession()
+		defer s.Close()
+		count, err := s.Where("created_by_id = ? AND title = ?", u.ID, "Mealie").Count(&models.Label{})
+		require.NoError(t, err)
+		assert.Equal(t, int64(1), count, "second import must reuse the existing 'Mealie' label")
+	})
+	t.Run("does not merge into another user's label", func(t *testing.T) {
+		db.LoadAndAssertFixtures(t)
+
+		// Fixture label #3 'Label #3 - other user' is created_by_id: 2.
+		// Importing the same title for user 1 must create a new, user-owned label.
+		structure := []*models.ProjectWithTasksAndBuckets{
+			{
+				Project: models.Project{Title: "Import project"},
+				Tasks: []*models.TaskWithComments{
+					{
+						Task: models.Task{
+							Title:  "Task",
+							Labels: []*models.Label{{Title: "Label #3 - other user"}},
+						},
+					},
+				},
+			},
+		}
+		require.NoError(t, InsertFromStructure(structure, u))
+
+		db.AssertExists(t, "labels", map[string]interface{}{
+			"title":         "Label #3 - other user",
+			"created_by_id": u.ID,
+		}, false)
+	})
 }