Fail if commit is already signed

Author: cruessler

gitoxide-core/src/repository/commit.rs

@@ -90,8 +90,14 @@ pub fn sign(repo: gix::Repository, rev_spec: Option<&str>, mut out: impl std::io
 
     let extra_header: Cow<'_, BStr> = Cow::Borrowed(signed_data.as_bstr());
 
-    // TODO:
-    // What do we want to do when there is already a signature?
+    assert!(
+        !commit_ref
+            .extra_headers
+            .iter()
+            .any(|(header_name, _)| *header_name == BStr::new(SIGNATURE_FIELD_NAME)),
+        "Commit is already signed, doing nothing"
+    );
+
     commit_ref
         .extra_headers
         .push((BStr::new(SIGNATURE_FIELD_NAME), extra_header));