diff --git a/datatree/datatree.py b/datatree/datatree.py
index 35830fa9..8ccd6c8b 100644
--- a/datatree/datatree.py
+++ b/datatree/datatree.py
@@ -125,6 +125,12 @@ def ds(self, data: Union[Dataset, DataArray] = None):
             )
         if isinstance(data, DataArray):
             data = data.to_dataset()
+        if data is not None:
+            for var in list(data.variables):
+                if var in list(c.name for c in self.children):
+                    raise KeyError(
+                        f"Cannot add variable named {var}: node already has a child named {var}"
+                    )
         self._ds = data
 
     @property
@@ -165,6 +171,30 @@ def _init_single_datatree_node(
         obj.ds = data
         return obj
 
+    def _pre_attach(self, parent: TreeNode) -> None:
+        """
+        Method which superclass calls before setting parent, here used to prevent having two
+        children with duplicate names (or a data variable with the same name as a child).
+        """
+        super()._pre_attach(parent)
+        if parent.has_data and self.name in list(parent.ds.variables):
+            raise KeyError(
+                f"parent {parent.name} already contains a data variable named {self.name}"
+            )
+
+    def add_child(self, child: TreeNode) -> None:
+        """
+        Add a single child node below this node, without replacement.
+
+        Will raise a KeyError if either a child or data variable already exists with this name.
+        """
+        if child.name in list(c.name for c in self.children):
+            raise KeyError(f"Node already has a child named {child.name}")
+        elif self.has_data and child.name in list(self.ds.variables):
+            raise KeyError(f"Node already contains a data variable named {child.name}")
+        else:
+            child.parent = self
+
     def __str__(self):
         """A printable representation of the structure of this entire subtree."""
         renderer = anytree.RenderTree(self)
diff --git a/datatree/tests/test_datatree.py b/datatree/tests/test_datatree.py
index df73109a..83e77710 100644
--- a/datatree/tests/test_datatree.py
+++ b/datatree/tests/test_datatree.py
@@ -90,6 +90,34 @@ def test_has_data(self):
         assert not john.has_data
 
 
+class TestVariablesChildrenNameCollisions:
+    def test_parent_already_has_variable_with_childs_name(self):
+        dt = DataNode("root", data=xr.Dataset({"a": [0], "b": 1}))
+        with pytest.raises(KeyError, match="already contains a data variable named a"):
+            DataNode("a", data=None, parent=dt)
+
+        with pytest.raises(KeyError, match="already contains a data variable named a"):
+            dt.add_child(DataNode("a", data=None))
+
+    def test_assign_when_already_child_with_variables_name(self):
+        dt = DataNode("root", data=None)
+        DataNode("a", data=None, parent=dt)
+        with pytest.raises(KeyError, match="already has a child named a"):
+            dt.ds = xr.Dataset({"a": 0})
+
+        dt.ds = xr.Dataset()
+        with pytest.raises(KeyError, match="already has a child named a"):
+            dt.ds = dt.ds.assign(a=xr.DataArray(0))
+
+    @pytest.mark.xfail
+    def test_update_when_already_child_with_variables_name(self):
+        # See issue #38
+        dt = DataNode("root", data=None)
+        DataNode("a", data=None, parent=dt)
+        with pytest.raises(KeyError, match="already has a child named a"):
+            dt.ds["a"] = xr.DataArray(0)
+
+
 class TestGetItems:
     def test_get_node(self):
         folder1 = DataNode("folder1")