[BUG] Don't overwrite protected branch accidentally

- If a user tries to create another protected branching rule that
specifies a set of branches already used by another rule, do not allow
it.
- Update the translation accordingly.
- Adds integration test.
- Resolves #2455

routers/web/repo/setting/protected_branch.go

@@ -132,12 +132,15 @@ func SettingsProtectedBranchPost(ctx *context.Context) {
 			}
 		}
 	} else {
-		// FIXME: If a new ProtectBranch has a duplicate RuleName, an error should be returned.
-		// Currently, if a new ProtectBranch with a duplicate RuleName is created, the existing ProtectBranch will be updated.
-		// But we cannot modify this logic now because many unit tests rely on it.
+		// Check if a rule already exists with this rulename, if so redirect to it.
 		protectBranch, err = git_model.GetProtectedBranchRuleByName(ctx, ctx.Repo.Repository.ID, f.RuleName)
 		if err != nil {
-			ctx.ServerError("GetProtectBranchOfRepoByName", err)
+			ctx.ServerError("GetProtectedBranchRuleByName", err)
+			return
+		}
+		if protectBranch != nil {
+			ctx.Flash.Error(ctx.Tr("repo.settings.protected_branch_duplicate_rule_name"))
+			ctx.Redirect(fmt.Sprintf("%s/settings/branches/edit?rule_name=%s", ctx.Repo.RepoLink, protectBranch.RuleName))
 			return
 		}
 	}