Distinguish never spawned from suicided before submit.

cylc/flow/task_pool.py

@@ -1564,8 +1564,8 @@ def _get_task_history(
         try:
             submit_num: int = max(s[0] for s in info)
         except ValueError:
-            # never spawned before in any flow
-            submit_num = 0
+            # never spawned in any flow
+            submit_num = -1
 
         prev_status: str = TASK_STATUS_WAITING
         prev_flow_wait = False
@@ -1623,6 +1623,18 @@ def spawn_task(
             self._get_task_history(name, point, flow_nums)
         )
 
+        if submit_num == 0:
+            # Spawned but suicided before it submitted.
+            # TODO - if `cylc remove` should run again?
+            # TODO - There's still a timing issue for suicide here!
+            # (need sleep 2 in task b in the example)
+            LOG.critical(f"Not spawning {point}/{name} because reasons")
+            return None
+
+        if submit_num == -1:
+            # Never spawned before - good.
+            submit_num = 0
+
         itask = self._get_task_proxy_db_outputs(
             point,
             self.config.get_taskdef(name),