Add note about features with the same id

src/ol/format/geojson.js

@@ -329,7 +329,8 @@ ol.format.GeoJSON.GEOMETRY_WRITERS_ = {
 /**
  * Read a feature from a GeoJSON Feature source.  Only works for Feature or
  * geometry types.  Use {@link ol.format.GeoJSON#readFeatures} to read
- * FeatureCollection source.
+ * FeatureCollection source. If feature at source has an id, it will be used
+ * as Feature id by calling {@link ol.Feature#setId} internally.
  *
  * @function
  * @param {Document|Node|Object|string} source Source.

src/ol/source/vector.js

@@ -157,7 +157,9 @@ ol.inherits(ol.source.Vector, ol.source.Source);
 /**
  * Add a single feature to the source.  If you want to add a batch of features
  * at once, call {@link ol.source.Vector#addFeatures source.addFeatures()}
- * instead.
+ * instead. A feature will not be added to the source if feature with
+ * the same id is already there. The reason for this behavior is to avoid
+ * feature duplication when using bbox or tile loading strategies.
  * @param {ol.Feature} feature Feature to add.
  * @api
  */