diff --git a/fmt/src/config/mod.rs b/fmt/src/config/mod.rs
index 6333124..46a5a4f 100644
--- a/fmt/src/config/mod.rs
+++ b/fmt/src/config/mod.rs
@@ -21,12 +21,11 @@ use std::path::PathBuf;
 use serde::de::Error;
 use serde::Deserialize;
 use serde::Deserializer;
-use serde::Serialize;
 use toml::Value;
 
 use crate::default_true;
 
-#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+#[derive(Debug, Clone, Default, Deserialize)]
 #[serde(default, rename_all = "camelCase")]
 pub struct Config {
     #[serde(default = "default_cwd")]
@@ -57,7 +56,7 @@ pub struct Config {
     pub additional_headers: Vec<String>,
 }
 
-#[derive(Debug, Clone, Copy, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Deserialize)]
 pub struct Git {
     pub attrs: FeatureGate,
     pub ignore: FeatureGate,
@@ -72,7 +71,7 @@ impl Default for Git {
     }
 }
 
-#[derive(Debug, Clone, Copy, Ord, PartialOrd, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, Ord, PartialOrd, Eq, PartialEq, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum FeatureGate {
     /// Determinate whether turn on the feature.
@@ -109,7 +108,7 @@ impl FeatureGate {
     }
 }
 
-#[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Eq, PartialEq)]
 pub enum Mapping {
     Filename {
         pattern: String,
diff --git a/fmt/src/processor.rs b/fmt/src/processor.rs
index 99ee01c..673734c 100644
--- a/fmt/src/processor.rs
+++ b/fmt/src/processor.rs
@@ -82,7 +82,11 @@ pub fn check_license_header<C: Callback>(
         let mut mapping = config.mapping.clone();
         if config.use_default_mapping {
             let default_mapping = default_mapping();
-            mapping.extend(default_mapping);
+            for m in default_mapping {
+                if !mapping.contains(&m) {
+                    mapping.insert(m);
+                }
+            }
         }
         mapping
     };
diff --git a/tests/bom_issue/licenserc.toml b/tests/bom_issue/licenserc.toml
index 767fc59..3eeab3c 100644
--- a/tests/bom_issue/licenserc.toml
+++ b/tests/bom_issue/licenserc.toml
@@ -7,5 +7,5 @@ additionalHeaders = [
     "style.toml"
 ]
 
-[mapping.HASH_SOURCE_STYLE]
+[mapping.CS_TEST_STYLE]
 extensions = ["cs"]
diff --git a/tests/bom_issue/style.toml b/tests/bom_issue/style.toml
index 72d8013..f759213 100644
--- a/tests/bom_issue/style.toml
+++ b/tests/bom_issue/style.toml
@@ -1,4 +1,4 @@
-[HASH_SOURCE_STYLE]
+[CS_TEST_STYLE]
 firstLine = "// -------------------------------------------------------"
 endLine = "// -------------------------------------------------------\n"
 skipLinePattern = "^#!.*$"