diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 7dabfb37..74b0cd85 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -43,28 +43,28 @@ jobs:
         os: [ubuntu-latest]
         flags:
           - ""
-          - "--features debugging"
-          - "--features metadata,serde,internals"
-          - "--features unchecked,serde,metadata,internals,debugging"
-          - "--features sync,serde,metadata,internals,debugging"
-          - "--features no_position,serde,metadata,internals,debugging"
-          - "--features no_optimize,serde,metadata,internals,debugging"
-          - "--features no_float,serde,metadata,internals,debugging"
-          - "--features f32_float,serde,metadata,internals,debugging"
-          - "--features decimal,serde,metadata,internals,debugging"
-          - "--features no_custom_syntax,serde,metadata,internals,debugging"
-          - "--features no_float,decimal"
-          - "--tests --features only_i32,serde,metadata,internals,debugging"
-          - "--features only_i64,serde,metadata,internals,debugging"
-          - "--features no_index,serde,metadata,internals,debugging"
-          - "--features no_object,serde,metadata,internals,debugging"
-          - "--features no_function,serde,metadata,internals,debugging"
-          - "--features no_module,serde,metadata,internals,debugging"
-          - "--features no_time,serde,metadata,internals,debugging"
-          - "--features no_closure,serde,metadata,internals,debugging"
-          - "--features unicode-xid-ident,serde,metadata,internals,debugging"
-          - "--features sync,no_time,no_function,no_float,no_position,no_optimize,no_module,no_closure,no_custom_syntax,metadata,serde,unchecked,debugging"
-          - "--features no_time,no_function,no_float,no_position,no_index,no_object,no_optimize,no_module,no_closure,no_custom_syntax,unchecked"
+          - "--features testing-environ,debugging"
+          - "--features testing-environ,metadata,serde,internals"
+          - "--features testing-environ,unchecked,serde,metadata,internals,debugging"
+          - "--features testing-environ,sync,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_position,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_optimize,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_float,serde,metadata,internals,debugging"
+          - "--features testing-environ,f32_float,serde,metadata,internals,debugging"
+          - "--features testing-environ,decimal,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_custom_syntax,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_float,decimal"
+          - "--tests --features testing-environ,only_i32,serde,metadata,internals,debugging"
+          - "--features testing-environ,only_i64,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_index,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_object,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_function,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_module,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_time,serde,metadata,internals,debugging"
+          - "--features testing-environ,no_closure,serde,metadata,internals,debugging"
+          - "--features testing-environ,unicode-xid-ident,serde,metadata,internals,debugging"
+          - "--features testing-environ,sync,no_time,no_function,no_float,no_position,no_optimize,no_module,no_closure,no_custom_syntax,metadata,serde,unchecked,debugging"
+          - "--features testing-environ,no_time,no_function,no_float,no_position,no_index,no_object,no_optimize,no_module,no_closure,no_custom_syntax,unchecked"
         toolchain: [stable]
         experimental: [false]
         include:
diff --git a/Cargo.toml b/Cargo.toml
index 06c4ec6a..9730399c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -117,6 +117,11 @@ std = ["ahash/std", "num-traits/std", "smartstring/std"]
     ## Use [`stdweb`](https://crates.io/crates/stdweb) as JavaScript interface.
     stdweb = ["getrandom/js", "instant/stdweb"]
 
+#! ### Features used in testing environments only
+
+    ## Running under a testing environment.
+    testing-environ = []
+
 [[bin]]
 name = "rhai-repl"
 required-features = ["rustyline"]
diff --git a/src/module/mod.rs b/src/module/mod.rs
index 89ef6152..961b4ed6 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -715,8 +715,24 @@ impl Module {
         // None + function name + number of arguments.
         let num_params = fn_def.params.len();
         let hash_script = crate::calc_fn_hash(None, &fn_def.name, num_params);
+
+        #[cfg(feature = "testing-environ")]
+        if let Some(f) = self
+            .functions
+            .get_or_insert_with(|| {
+                StraightHashMap::with_capacity_and_hasher(FN_MAP_SIZE, Default::default())
+            })
+            .get(&hash_script)
+        {
+            panic!(
+                "PANIC ATTACK!!! Function hash {} already exists when registering function {:#?}:\n{:#?}",
+                hash_script, fn_def, f
+            );
+        }
+
         #[cfg(feature = "metadata")]
         let params_info = fn_def.params.iter().map(Into::into).collect();
+
         self.functions
             .get_or_insert_with(|| {
                 StraightHashMap::with_capacity_and_hasher(FN_MAP_SIZE, Default::default())
@@ -1046,6 +1062,20 @@ impl Module {
         let hash_script = calc_fn_hash(None, name, param_types.len());
         let hash_fn = calc_fn_hash_full(hash_script, param_types.iter().copied());
 
+        #[cfg(feature = "testing-environ")]
+        if let Some(f) = self
+            .functions
+            .get_or_insert_with(|| {
+                StraightHashMap::with_capacity_and_hasher(FN_MAP_SIZE, Default::default())
+            })
+            .get(&hash_script)
+        {
+            panic!(
+                "PANIC ATTACK!!! Function hash {} already exists when registering function {}:\n{:#?}",
+                hash_script, name, f
+            );
+        }
+
         if is_dynamic {
             self.dynamic_functions_filter
                 .get_or_insert_with(Default::default)