diff --git a/goalc/build_level/jak1/build_level.cpp b/goalc/build_level/jak1/build_level.cpp
index 1c79972879..4a6b4d534c 100644
--- a/goalc/build_level/jak1/build_level.cpp
+++ b/goalc/build_level/jak1/build_level.cpp
@@ -48,6 +48,12 @@ bool run_build_level(const std::string& input_file,
   auto dts = decompiler::DecompilerTypeSystem(GameVersion::Jak1);
   dts.parse_enum_defs({"decompiler", "config", "jak1", "all-types.gc"});
   add_actors_from_json(level_json.at("actors"), actors, level_json.value("base_id", 1234), dts);
+  std::sort(actors.begin(), actors.end(), [](auto& a, auto& b) { return a.aid < b.aid; });
+  auto duplicates = std::adjacent_find(actors.begin(), actors.end(),
+                                       [](auto& a, auto& b) { return a.aid == b.aid; });
+  ASSERT_MSG(duplicates == actors.end(),
+             fmt::format("Actor IDs must be unique. Found at least two actors with ID {}",
+                         duplicates->aid));
   file.actors = std::move(actors);
   // ambients
   std::vector<EntityAmbient> ambients;
diff --git a/goalc/build_level/jak2/build_level.cpp b/goalc/build_level/jak2/build_level.cpp
index 44bb3e05d6..2e418e8bb7 100644
--- a/goalc/build_level/jak2/build_level.cpp
+++ b/goalc/build_level/jak2/build_level.cpp
@@ -47,6 +47,12 @@ bool run_build_level(const std::string& input_file,
   dts.parse_enum_defs({"decompiler", "config", "jak2", "all-types.gc"});
   std::vector<EntityActor> actors;
   add_actors_from_json(level_json.at("actors"), actors, level_json.value("base_id", 1234), dts);
+  std::sort(actors.begin(), actors.end(), [](auto& a, auto& b) { return a.aid < b.aid; });
+  auto duplicates = std::adjacent_find(actors.begin(), actors.end(),
+                                       [](auto& a, auto& b) { return a.aid == b.aid; });
+  ASSERT_MSG(duplicates == actors.end(),
+             fmt::format("Actor IDs must be unique. Found at least two actors with ID {}",
+                         duplicates->aid));
   file.actors = std::move(actors);
   // cameras
   // nodes