diff --git a/crates/clawhdf5-format/src/file_writer.rs b/crates/clawhdf5-format/src/file_writer.rs
index 0b318a9..11345a1 100644
--- a/crates/clawhdf5-format/src/file_writer.rs
+++ b/crates/clawhdf5-format/src/file_writer.rs
@@ -1355,7 +1355,10 @@ fn flatten_ds(db: DatasetBuilder, refcount: u32) -> Result<DsFlat, FormatError>
             timestamp: prov.timestamp.clone(),
             source: prov.source.clone(),
         };
-        attrs.extend(p.build_attrs(&raw));
+        // The provenance attributes replace any the caller set by hand.
+        let prov = p.build_attrs(&raw);
+        attrs.retain(|a| prov.iter().all(|b| b.name != a.name));
+        attrs.extend(prov);
     }
     let fill_message = fill_value_message(db.fill_time, db.fill_value.as_deref(), &dt)?;
     Ok(DsFlat {
diff --git a/crates/clawhdf5-format/src/type_builders.rs b/crates/clawhdf5-format/src/type_builders.rs
index fa64465..ce43f64 100644
--- a/crates/clawhdf5-format/src/type_builders.rs
+++ b/crates/clawhdf5-format/src/type_builders.rs
@@ -695,8 +695,13 @@ impl DatasetBuilder {
         self
     }
 
+    /// Set attribute `name`. Setting it again replaces the earlier value,
+    /// as `attrs[name] = v` does in h5py.
     pub fn set_attr(&mut self, name: &str, value: AttrValue) -> &mut Self {
-        self.attrs.push((name.to_string(), value));
+        match self.attrs.iter_mut().find(|(n, _)| n == name) {
+            Some(slot) => slot.1 = value,
+            None => self.attrs.push((name.to_string(), value)),
+        }
         self
     }
 
diff --git a/crates/clawhdf5/tests/writer_groups_interop.rs b/crates/clawhdf5/tests/writer_groups_interop.rs
index e8488c9..97e3544 100644
--- a/crates/clawhdf5/tests/writer_groups_interop.rs
+++ b/crates/clawhdf5/tests/writer_groups_interop.rs
@@ -876,3 +876,65 @@ fn chained_hard_links_resolve_in_linear_time() {
         assert_eq!(f.dataset("g/s59/s0/v").unwrap().read_i32().unwrap(), [5]);
     }
 }
+
+#[test]
+fn a_dataset_attribute_set_again_takes_the_new_value() {
+    skip_if_no_python!();
+    // Setting a dataset attribute twice wrote two attribute messages with
+    // one name, and h5py read back the first value. Also with dense
+    // attribute storage (more than 8).
+    let dir = tempfile::tempdir().unwrap();
+    let mut b = FileBuilder::new();
+    b.create_dataset("x")
+        .with_f64_data(&[1.0])
+        .set_attr("a", AttrValue::I64(1))
+        .set_attr("a", AttrValue::I64(2));
+    let d = b.create_dataset("dense");
+    d.with_i32_data(&[1]);
+    for i in 0..12 {
+        d.set_attr(&format!("k{i:02}"), AttrValue::I64(i));
+    }
+    d.set_attr("k03", AttrValue::String("three".into()));
+    let path = write(&dir, "ds_attrs.h5", b);
+    let out = h5py(
+        &path,
+        "with h5py.File(path, 'r') as f:\n\
+         \x20   a, d = f['x'].attrs, f['dense'].attrs\n\
+         \x20   print(json.dumps([list(a), int(a['a']), len(d), d['k03'].decode(), int(d['k04'])]))",
+    );
+    assert_eq!(out, r#"[["a"], 2, 12, "three", 4]"#);
+    let f = File::open(&path).unwrap();
+    assert!(matches!(
+        f.dataset("x").unwrap().attrs().unwrap()["a"],
+        AttrValue::I64(2)
+    ));
+}
+
+#[cfg(feature = "provenance")]
+#[test]
+fn provenance_attributes_replace_ones_set_by_hand() {
+    skip_if_no_python!();
+    // A hand-set attribute with a provenance attribute's name was written
+    // next to the computed one, and h5py read the hand-set value.
+    let dir = tempfile::tempdir().unwrap();
+    let mut b = FileBuilder::new();
+    b.create_dataset("p")
+        .with_i32_data(&[1, 2])
+        .with_provenance("me", "2026-09-26T00:00:00Z", None)
+        .set_attr("_provenance_sha256", AttrValue::String("forged".into()));
+    let path = write(&dir, "prov.h5", b);
+    let out = h5py(
+        &path,
+        "with h5py.File(path, 'r') as f:\n\
+         \x20   a = f['p'].attrs\n\
+         \x20   h = a['_provenance_sha256']\n\
+         \x20   h = h.decode() if isinstance(h, bytes) else h\n\
+         \x20   print(json.dumps([list(a).count('_provenance_sha256'), h != 'forged']))",
+    );
+    assert_eq!(out, "[1, true]");
+    let f = File::open(&path).unwrap();
+    assert_eq!(
+        f.dataset("p").unwrap().verify_provenance().unwrap(),
+        clawhdf5_format::provenance::VerifyResult::Ok
+    );
+}