Check to make sure Feature isn't present before adding it (#4323)

(cherry picked from commit b1c29d8)

fabric-biome-api-v1/src/main/java/net/fabricmc/fabric/impl/biome/modification/BiomeModificationContextImpl.java

@@ -276,7 +276,14 @@ public void addFeature(GenerationStep.Feature step, RegistryKey<PlacedFeature> e
 				featureSteps.add(RegistryEntryList.of(Collections.emptyList()));
 			}
 
-			featureSteps.set(index, plus(featureSteps.get(index), getEntry(features, entry)));
+			RegistryEntry.Reference<PlacedFeature> feature = getEntry(features, entry);
+
+			// Don't add the feature if it's already present
+			if (featureSteps.get(index).contains(feature)) {
+				return;
+			}
+
+			featureSteps.set(index, plus(featureSteps.get(index), feature));
 
 			// Ensure the list of flower features is up-to-date
 			rebuildFeatures = true;