Prevent to create duplicated pull request

options/locale/locale_en-US.ini

@@ -1862,6 +1862,7 @@ pulls.desc = Enable pull requests and code reviews.
 pulls.new = New Pull Request
 pulls.new.blocked_user = Cannot create pull request because you are blocked by the repository owner.
 pulls.new.must_collaborator = You must be a collaborator to create pull request.
+pulls.new.already_existed = A pull request between these branches already exists
 pulls.edit.already_changed = Unable to save changes to the pull request. It appears the content has already been changed by another user. Please refresh the page and try editing again to avoid overwriting their changes.
 pulls.view = View Pull Request
 pulls.compare_changes = New Pull Request

routers/web/repo/pull.go

@@ -1340,6 +1340,17 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 		return
 	}
 
+	// Check if a pull request already exists with the same head and base branch.
+	pr, err := issues_model.GetUnmergedPullRequest(ctx, ci.HeadRepo.ID, repo.ID, ci.HeadBranch, ci.BaseBranch, issues_model.PullRequestFlowGithub)
+	if err != nil && !issues_model.IsErrPullRequestNotExist(err) {
+		ctx.ServerError("GetUnmergedPullRequest", err)
+		return
+	}
+	if pr != nil {
+		ctx.JSONError(ctx.Tr("repo.pulls.new.already_existed"))
+		return
+	}
+
 	content := form.Content
 	if filename := ctx.Req.Form.Get("template-file"); filename != "" {
 		if template, err := issue_template.UnmarshalFromRepo(ctx.Repo.GitRepo, ctx.Repo.Repository.DefaultBranch, filename); err == nil {

tests/integration/pull_create_test.go

@@ -156,8 +156,16 @@ func TestPullCreate(t *testing.T) {
 		url := test.RedirectURL(resp)
 		assert.Regexp(t, "^/user2/repo1/pulls/[0-9]*$", url)
 
+		// test create the pull request again and it should fail now
+		link := "/user2/repo1/compare/master...user1/repo1:master"
+		req := NewRequestWithValues(t, "POST", link, map[string]string{
+			"_csrf": GetUserCSRFToken(t, session),
+			"title": "This is a pull title",
+		})
+		session.MakeRequest(t, req, http.StatusBadRequest)
+
 		// check .diff can be accessed and matches performed change
-		req := NewRequest(t, "GET", url+".diff")
+		req = NewRequest(t, "GET", url+".diff")
 		resp = session.MakeRequest(t, req, http.StatusOK)
 		assert.Regexp(t, `\+Hello, World \(Edited\)`, resp.Body)
 		assert.Regexp(t, "^diff", resp.Body)