Added a functional test.

tests/functional/reload/28-preparing.t

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+# 
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#-------------------------------------------------------------------------------
+
+# Test for duplicate job submissions when preparing tasks get flushed
+# prior to reload - see https://github.com/cylc/cylc-flow/pull/6345
+
+. "$(dirname "$0")/test_header"
+set_test_number 4
+
+# Strap the process pool size down to 1, so that the first task is stuck
+# in the preparing state until the startup event handler finishes.
+
+create_test_global_config "" "
+[scheduler]
+    process pool size = 1
+"
+
+# install and play the workflow, then reload it and wait for it to finish.
+install_workflow "${TEST_NAME_BASE}" "${TEST_NAME_BASE}"
+run_ok "${TEST_NAME_BASE}-vip" cylc validate "${WORKFLOW_NAME}"
+run_ok "${TEST_NAME_BASE}-reload" cylc play "${WORKFLOW_NAME}"
+run_ok "${TEST_NAME_BASE}-reload" cylc reload "${WORKFLOW_NAME}"
+poll_grep_workflow_log -F 'INFO - DONE'
+
+# check that task `foo` was only submitted once.
+count_ok "1/foo.*submitted to" "${WORKFLOW_RUN_DIR}/log/scheduler/log" 1
+
+purge

tests/functional/reload/28-preparing/flow.cylc

@@ -0,0 +1,8 @@
+[scheduler]
+   [[events]]
+       startup handlers = "sleep 10; echo"
+[scheduling]
+    [[graph]]
+        R1 = "foo => bar"
+[runtime]
+    [[foo, bar]]