duplicate the possible result and add tests

models/migrations/fixtures/Test_AddCombinedIndexToIssueUser/issue_user.yml

@@ -0,0 +1,20 @@
+-
+  id: 1
+  uid: 1
+  issue_id: 1
+  is_read: true
+  is_mentioned: false
+
+-
+  id: 2
+  uid: 2
+  issue_id: 1
+  is_read: true
+  is_mentioned: false
+
+-
+  id: 3
+  uid: 2
+  issue_id: 1 # duplicated with id 2
+  is_read: false
+  is_mentioned: true

models/migrations/v1_22/main_test.go

@@ -0,0 +1,14 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_22 //nolint
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+)
+
+func TestMain(m *testing.M) {
+	base.MainTest(m)
+}

models/migrations/v1_22/v283.go

@@ -8,6 +8,23 @@ import (
 )
 
 func AddCombinedIndexToIssueUser(x *xorm.Engine) error {
+	type OldIssueUser struct {
+		IssueID int64
+		UID     int64
+		Cnt     int64
+	}
+
+	var duplicatedIssueUsers []OldIssueUser
+	if err := x.SQL("select * from (select issue_id, uid, count(1) as cnt from issue_user group by issue_id, uid) a where a.cnt > 1").
+		Find(&duplicatedIssueUsers); err != nil {
+		return err
+	}
+	for _, issueUser := range duplicatedIssueUsers {
+		if _, err := x.Exec("delete from issue_user where id in (SELECT id FROM issue_user WHERE issue_id = ? and uid = ? limit ?)", issueUser.IssueID, issueUser.UID, issueUser.Cnt-1); err != nil {
+			return err
+		}
+	}
+
 	type IssueUser struct {
 		UID     int64 `xorm:"INDEX unique(uid_to_issue)"` // User ID.
 		IssueID int64 `xorm:"INDEX unique(uid_to_issue)"`

models/migrations/v1_22/v283_test.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_22 //nolint
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/migrations/base"
+)
+
+func Test_AddCombinedIndexToIssueUser(t *testing.T) {
+	type IssueUser struct {
+		UID     int64 `xorm:"INDEX unique(uid_to_issue)"` // User ID.
+		IssueID int64 `xorm:"INDEX unique(uid_to_issue)"`
+	}
+
+	// Prepare and load the testing database
+	x, deferable := base.PrepareTestEnv(t, 0, new(IssueUser))
+	defer deferable()
+	if x == nil || t.Failed() {
+		return
+	}
+
+	if err := AddCombinedIndexToIssueUser(x); err != nil {
+		t.Fatal(err)
+	}
+}