diff --git a/python/ray/tune/ray_trial_executor.py b/python/ray/tune/ray_trial_executor.py
index 52b8c63f5..86cb6d5a7 100644
--- a/python/ray/tune/ray_trial_executor.py
+++ b/python/ray/tune/ray_trial_executor.py
@@ -19,6 +19,7 @@ from ray.tune.trial import Trial, Checkpoint, Location
 from ray.tune.resources import Resources
 from ray.tune.trial_executor import TrialExecutor
 from ray.tune.util import warn_if_slow
+from ray.tune.error import TuneError
 
 logger = logging.getLogger(__name__)
 
@@ -126,6 +127,21 @@ class RayTrialExecutor(TrialExecutor):
     def _train(self, trial):
         """Start one iteration of training and save remote id."""
 
+        if self._find_item(self._paused, trial):
+            raise TuneError(
+                "Should not call `train` on PAUSED trial {}. "
+                "This is an internal error - please file an issue "
+                "on https://github.com/ray-project/ray/issues/.".format(
+                    str(trial)))
+
+        if self._find_item(self._running, trial):
+            logging.debug(
+                "Trial {} already has a queued future. Skipping this "
+                "`train` call. This may occur if a trial has "
+                "been unpaused within a scheduler callback.".format(
+                    str(trial)))
+            return
+
         assert trial.status == Trial.RUNNING, trial.status
         remote = trial.runner.train.remote()
 
@@ -134,6 +150,8 @@ class RayTrialExecutor(TrialExecutor):
             remote = _LocalWrapper(remote)
 
         self._running[remote] = trial
+        trial_item = self._find_item(self._running, trial)
+        assert len(trial_item) < 2, trial_item
 
     def _start_trial(self, trial, checkpoint=None, runner=None):
         """Starts trial and restores last result if trial was paused.
@@ -308,7 +326,7 @@ class RayTrialExecutor(TrialExecutor):
                     trainable.reset_config.remote(new_config),
                     DEFAULT_GET_TIMEOUT)
             except RayTimeoutError:
-                logger.exception("Trial %s: reset_config timed out.")
+                logger.exception("Trial %s: reset_config timed out.", trial)
                 return False
         return reset_val