diff --git a/models/migrations/fixtures/Test_AddUniqueIndexForProjectIssue/project_issue.yml b/models/migrations/fixtures/Test_AddUniqueIndexForProjectIssue/project_issue.yml
new file mode 100644
index 0000000000000..6feaeb39f0598
--- /dev/null
+++ b/models/migrations/fixtures/Test_AddUniqueIndexForProjectIssue/project_issue.yml
@@ -0,0 +1,9 @@
+-
+  id: 1
+  project_id: 1
+  issue_id: 1
+
+-
+  id: 2
+  project_id: 1
+  issue_id: 1
diff --git a/models/migrations/migrations.go b/models/migrations/migrations.go
index 0daa799ff6754..387cd96a53471 100644
--- a/models/migrations/migrations.go
+++ b/models/migrations/migrations.go
@@ -21,6 +21,7 @@ import (
 	"code.gitea.io/gitea/models/migrations/v1_20"
 	"code.gitea.io/gitea/models/migrations/v1_21"
 	"code.gitea.io/gitea/models/migrations/v1_22"
+	"code.gitea.io/gitea/models/migrations/v1_23"
 	"code.gitea.io/gitea/models/migrations/v1_6"
 	"code.gitea.io/gitea/models/migrations/v1_7"
 	"code.gitea.io/gitea/models/migrations/v1_8"
@@ -572,6 +573,10 @@ var migrations = []Migration{
 	NewMigration("Ensure every project has exactly one default column - No Op", noopMigration),
 	// v293 -> v294
 	NewMigration("Ensure every project has exactly one default column", v1_22.CheckProjectColumnsConsistency),
+
+	// Gitea 1.22.0 ends at 294
+
+	NewMigration("Add unique index for project issue table", v1_23.AddUniqueIndexForProjectIssue),
 }
 
 // GetCurrentDBVersion returns the current db version
diff --git a/models/migrations/v1_23/main_test.go b/models/migrations/v1_23/main_test.go
new file mode 100644
index 0000000000000..b7948bd4dd248
--- /dev/null
+++ b/models/migrations/v1_23/main_test.go
@@ -0,0 +1,14 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_23 //nolint
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+)
+
+func TestMain(m *testing.M) {
+	base.MainTest(m)
+}
diff --git a/models/migrations/v1_23/v294.go b/models/migrations/v1_23/v294.go
new file mode 100644
index 0000000000000..f2a54f6d23ecc
--- /dev/null
+++ b/models/migrations/v1_23/v294.go
@@ -0,0 +1,53 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_23 //nolint
+
+import (
+	"fmt"
+
+	"xorm.io/xorm"
+	"xorm.io/xorm/schemas"
+)
+
+// AddUniqueIndexForProjectIssue adds unique indexes for project issue table
+func AddUniqueIndexForProjectIssue(x *xorm.Engine) error {
+	// remove possible duplicated records in table project_issue
+	type result struct {
+		IssueID   int64
+		ProjectID int64
+		Cnt       int
+	}
+	var results []result
+	if err := x.Select("issue_id, project_id, count(*) as cnt").
+		Table("project_issue").
+		GroupBy("issue_id, project_id").
+		Having("count(*) > 1").
+		Find(&results); err != nil {
+		return err
+	}
+	for _, r := range results {
+		if x.Dialect().URI().DBType == schemas.MSSQL {
+			if _, err := x.Exec(fmt.Sprintf("delete from project_issue where id in (SELECT top %d id FROM project_issue WHERE issue_id = ? and project_id = ?)", r.Cnt-1), r.IssueID, r.ProjectID); err != nil {
+				return err
+			}
+		} else {
+			var ids []int64
+			if err := x.SQL("SELECT id FROM project_issue WHERE issue_id = ? and project_id = ? limit ?", r.IssueID, r.ProjectID, r.Cnt-1).Find(&ids); err != nil {
+				return err
+			}
+			if _, err := x.Table("project_issue").In("id", ids).Delete(); err != nil {
+				return err
+			}
+		}
+	}
+
+	// add unique index for project_issue table
+	type ProjectIssue struct { //revive:disable-line:exported
+		ID        int64 `xorm:"pk autoincr"`
+		IssueID   int64 `xorm:"INDEX unique(s)"`
+		ProjectID int64 `xorm:"INDEX unique(s)"`
+	}
+
+	return x.Sync(new(ProjectIssue))
+}
diff --git a/models/migrations/v1_23/v294_test.go b/models/migrations/v1_23/v294_test.go
new file mode 100644
index 0000000000000..d9a44ad866bb1
--- /dev/null
+++ b/models/migrations/v1_23/v294_test.go
@@ -0,0 +1,52 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_23 //nolint
+
+import (
+	"slices"
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+
+	"github.com/stretchr/testify/assert"
+	"xorm.io/xorm/schemas"
+)
+
+func Test_AddUniqueIndexForProjectIssue(t *testing.T) {
+	type ProjectIssue struct { //revive:disable-line:exported
+		ID        int64 `xorm:"pk autoincr"`
+		IssueID   int64 `xorm:"INDEX"`
+		ProjectID int64 `xorm:"INDEX"`
+	}
+
+	// Prepare and load the testing database
+	x, deferable := base.PrepareTestEnv(t, 0, new(ProjectIssue))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	cnt, err := x.Table("project_issue").Where("project_id=1 AND issue_id=1").Count()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 2, cnt)
+
+	assert.NoError(t, AddUniqueIndexForProjectIssue(x))
+
+	cnt, err = x.Table("project_issue").Where("project_id=1 AND issue_id=1").Count()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, cnt)
+
+	tables, err := x.DBMetas()
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, len(tables))
+	found := false
+	for _, index := range tables[0].Indexes {
+		if index.Type == schemas.UniqueType {
+			found = true
+			slices.Equal(index.Cols, []string{"project_id", "issue_id"})
+			break
+		}
+	}
+	assert.True(t, found)
+}