--- a/net/minecraft/util/datafix/codec/DatapackCodec.java
+++ b/net/minecraft/util/datafix/codec/DatapackCodec.java
@@ -18,7 +18,7 @@
    private final List<String> field_234883_d_;
 
    public DatapackCodec(List<String> p_i231607_1_, List<String> p_i231607_2_) {
-      this.field_234882_c_ = ImmutableList.copyOf(p_i231607_1_);
+      this.field_234882_c_ = com.google.common.collect.Lists.newArrayList(p_i231607_1_);
       this.field_234883_d_ = ImmutableList.copyOf(p_i231607_2_);
    }
 
@@ -29,4 +29,8 @@
    public List<String> func_234887_b_() {
       return this.field_234883_d_;
    }
+
+   public void addModPacks(List<String> modPacks) {
+      field_234882_c_.addAll(modPacks.stream().filter(p->!field_234882_c_.contains(p)).collect(java.util.stream.Collectors.toList()));
+   }
 }
