diff --git a/geo_activity_playground/core/activities.py b/geo_activity_playground/core/activities.py
index 4244ecc..3b66868 100644
--- a/geo_activity_playground/core/activities.py
+++ b/geo_activity_playground/core/activities.py
@@ -48,6 +48,9 @@ def __len__(self) -> int:
         return len(self.meta)
 
     def add_activity(self, activity_meta: ActivityMeta) -> None:
+        assert not self.has_activity(
+            activity_meta["id"]
+        ), f"Trying to add the following activity which already exists: {activity_meta}"
         self._loose_activities.append(activity_meta)
 
     def commit(self) -> None:
@@ -95,7 +98,9 @@ def iter_activities(self, new_to_old=True) -> Iterator[ActivityMeta]:
 
     @functools.lru_cache()
     def get_activity_by_id(self, id: int) -> ActivityMeta:
-        return self.meta.loc[id]
+        activity = self.meta.loc[id]
+        assert isinstance(activity["name"], str), activity["name"]
+        return activity
 
     @functools.lru_cache(maxsize=3000)
     def get_time_series(self, id: int) -> pd.DataFrame:
diff --git a/geo_activity_playground/explorer/grid_file.py b/geo_activity_playground/explorer/grid_file.py
index fd8fc19..0e5c5eb 100644
--- a/geo_activity_playground/explorer/grid_file.py
+++ b/geo_activity_playground/explorer/grid_file.py
@@ -1,3 +1,4 @@
+import json
 import logging
 from typing import Iterator
 from typing import Optional
@@ -53,6 +54,11 @@ def make_explorer_rectangle(
             (x1, y1, zoom),
         ]
     ]
+    try:
+        json.dumps(properties)
+    except TypeError:
+        logger.error(f"Cannot serialize the following as JSON: {properties}")
+        raise
     return geojson.Feature(
         geometry=geojson.Polygon([[(coord[1], coord[0]) for coord in corners]]),
         properties=properties,
diff --git a/geo_activity_playground/importers/strava_api.py b/geo_activity_playground/importers/strava_api.py
index 5956306..528fec0 100644
--- a/geo_activity_playground/importers/strava_api.py
+++ b/geo_activity_playground/importers/strava_api.py
@@ -118,6 +118,9 @@ def try_import_strava(repository: ActivityRepository) -> None:
         for activity in tqdm(
             client.get_activities(after=get_after), desc="Downloading Strava activities"
         ):
+            # Sometimes we still get an activity here although it has already been imported from the Strava checkout.
+            if repository.has_activity(activity.id):
+                continue
             cache_file = (
                 pathlib.Path("Cache") / "Activity Metadata" / f"{activity.id}.pickle"
             )