diff --git a/topojson/core/extract.py b/topojson/core/extract.py
index e5539a5..9e8132f 100644
--- a/topojson/core/extract.py
+++ b/topojson/core/extract.py
@@ -470,8 +470,8 @@ def _extract_featurecollection(self, geom):
 
         # check for overwritten duplicate keys
         if len(data) < len(obj["features"]):
-            # slight problem here is it doesn't say which one/ones were duplicated
-            raise IndexError("id in geojson data duplicated")
+            msg = "index in data duplicated, use `ignore_index=True` to overwrite index"
+            raise IndexError(msg)
 
         # new data dictionary is created, throw the geometries back to main()
         self._is_single = False
diff --git a/topojson/core/topology.py b/topojson/core/topology.py
index 4e91d5b..a10d483 100644
--- a/topojson/core/topology.py
+++ b/topojson/core/topology.py
@@ -99,8 +99,8 @@ class Topology(Hashmap):
         combination with an equal length list of `data` objects.
         Default is a single object named `data`.
     ignore_index : bool
-        If set to true existing ids/indexes will be ignored and overwritten otherwise
-        they will be passed through into the output.
+        If set to true existing ids/indexes of geojson FeatureCollections will be
+        ignored and overwritten. Otherwise features with ids will use their existing one.
         If indexes are not ignored and a duplicate id exists an exception will be raised.
         Default is false.
     """