Error on duplicate PEP 735 dependency groups (#8390)

## Summary

Part of: https://github.com/astral-sh/uv/pull/8272.

crates/uv-workspace/src/pyproject.rs

@@ -46,7 +46,7 @@ pub struct PyProjectToml {
     /// Tool-specific metadata.
     pub tool: Option<Tool>,
     /// Non-project dependency groups, as defined in PEP 735.
-    pub dependency_groups: Option<BTreeMap<GroupName, Vec<DependencyGroupSpecifier>>>,
+    pub dependency_groups: Option<DependencyGroups>,
     /// The raw unserialized document.
     #[serde(skip)]
     pub raw: String,
@@ -540,6 +540,79 @@ impl Deref for SerdePattern {
     }
 }
 
+#[derive(Debug, Clone, PartialEq)]
+#[cfg_attr(test, derive(Serialize))]
+pub struct DependencyGroups(BTreeMap<GroupName, Vec<DependencyGroupSpecifier>>);
+
+impl DependencyGroups {
+    /// Returns the names of the dependency groups.
+    pub fn keys(&self) -> impl Iterator<Item = &GroupName> {
+        self.0.keys()
+    }
+
+    /// Returns the dependency group with the given name.
+    pub fn get(&self, group: &GroupName) -> Option<&Vec<DependencyGroupSpecifier>> {
+        self.0.get(group)
+    }
+
+    /// Returns an iterator over the dependency groups.
+    pub fn iter(&self) -> impl Iterator<Item = (&GroupName, &Vec<DependencyGroupSpecifier>)> {
+        self.0.iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a DependencyGroups {
+    type Item = (&'a GroupName, &'a Vec<DependencyGroupSpecifier>);
+    type IntoIter = std::collections::btree_map::Iter<'a, GroupName, Vec<DependencyGroupSpecifier>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.iter()
+    }
+}
+
+/// Ensure that all keys in the TOML table are unique.
+impl<'de> serde::de::Deserialize<'de> for DependencyGroups {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct GroupVisitor;
+
+        impl<'de> serde::de::Visitor<'de> for GroupVisitor {
+            type Value = DependencyGroups;
+
+            fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                formatter.write_str("a table with unique dependency group names")
+            }
+
+            fn visit_map<M>(self, mut access: M) -> Result<Self::Value, M::Error>
+            where
+                M: serde::de::MapAccess<'de>,
+            {
+                let mut sources = BTreeMap::new();
+                while let Some((key, value)) =
+                    access.next_entry::<GroupName, Vec<DependencyGroupSpecifier>>()?
+                {
+                    match sources.entry(key) {
+                        std::collections::btree_map::Entry::Occupied(entry) => {
+                            return Err(serde::de::Error::custom(format!(
+                                "duplicate dependency group: `{}`",
+                                entry.key()
+                            )));
+                        }
+                        std::collections::btree_map::Entry::Vacant(entry) => {
+                            entry.insert(value);
+                        }
+                    }
+                }
+                Ok(DependencyGroups(sources))
+            }
+        }
+
+        deserializer.deserialize_map(GroupVisitor)
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[serde(rename_all = "kebab-case", try_from = "SourcesWire")]

crates/uv/tests/it/lock.rs

@@ -16635,6 +16635,42 @@ fn lock_group_invalid_entry_group_name() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn lock_group_invalid_duplicate_group_name() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["typing-extensions"]
+
+        [dependency-groups]
+        foo-bar = []
+        foo_bar = []
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Failed to parse: `pyproject.toml`
+      Caused by: TOML parse error at line 8, column 9
+      |
+    8 |         [dependency-groups]
+      |         ^^^^^^^^^^^^^^^^^^^
+    duplicate dependency group: `foo-bar`
+    "###);
+
+    Ok(())
+}
+
 #[test]
 fn lock_group_invalid_entry_table() -> Result<()> {
     let context = TestContext::new("3.12");