Compare subproc queue items better.

cylc/flow/subprocctx.py

@@ -137,6 +137,19 @@ def __str__(self):
                     'mesg': mesg}
         return ret.rstrip()
 
+    def equal(self, other: 'SubProcContext') -> bool:
+        """Compare my input attributes with those of other.
+
+        Ignore timestaemp and result attributes.
+
+        """
+        return (
+            self.cmd_key == other.cmd_key and
+            self.cmd == other.cmd and
+            self.cmd_kwargs == other.cmd_kwargs and
+            self.host == other.host
+        )
+
 
 class SubFuncContext(SubProcContext):
     """Represent the context of a Python function to run as a subprocess.

cylc/flow/subprocpool.py

@@ -348,17 +348,23 @@ def put_command(
                 callback_255=callback_255, callback_255_args=callback_255_args
             )
         else:
+            queueit = [
+                ctx, bad_hosts, callback, callback_args,
+                callback_255, callback_255_args
+            ]
             if (
-                not any(iq[0].cmd == ctx.cmd for iq in self.queuings) and
-                not any(ir[1].cmd == ctx.cmd for ir in self.runnings)
+                not any(
+                    queueit[0].equal(iq[0]) and queueit[1:] == iq[1:]
+                    for iq in self.queuings
+                ) and not any(
+                    queueit[0].equal(ir[1]) and queueit[1:] == ir[2:]
+                    for ir in self.runnings
+                )
             ):
                 # Queue it if not already queued or running.
-                self.queuings.append(
-                    [
-                        ctx, bad_hosts, callback, callback_args,
-                        callback_255, callback_255_args
-                    ]
-                )
+                # (Compare all attributes of queued items except for
+                # ctx results and timestamps).
+                self.queuings.append(queueit)
 
     @classmethod
     def run_command(cls, ctx):