diff --git a/pywr-core/src/lib.rs b/pywr-core/src/lib.rs
index 2a6bcf01..7220c577 100644
--- a/pywr-core/src/lib.rs
+++ b/pywr-core/src/lib.rs
@@ -179,6 +179,8 @@ pub enum PywrError {
     NetworkNotFound(String),
     #[error("network index ({0}) not found")]
     NetworkIndexNotFound(usize),
+    #[error("network name `{0}` already exists.")]
+    NetworkNameAlreadyExists(String),
     #[error("parameters do not provide an initial value")]
     ParameterNoInitialValue,
     #[error("parameter state not found for parameter index {0}")]
diff --git a/pywr-core/src/models/multi.rs b/pywr-core/src/models/multi.rs
index a9568338..863295cb 100644
--- a/pywr-core/src/models/multi.rs
+++ b/pywr-core/src/models/multi.rs
@@ -114,8 +114,12 @@ impl MultiNetworkModel {
             .ok_or(PywrError::NetworkNotFound(name.to_string()))
     }
 
-    pub fn add_network(&mut self, name: &str, network: Network) -> usize {
-        // TODO check for duplicate names
+    /// Add a [`Network`] to the model. The name must be unique.
+    pub fn add_network(&mut self, name: &str, network: Network) -> Result<usize, PywrError> {
+        if self.get_network_index_by_name(name).is_ok() {
+            return Err(PywrError::NetworkNameAlreadyExists(name.to_string()));
+        }
+
         let idx = self.networks.len();
         self.networks.push(MultiNetworkEntry {
             name: name.to_string(),
@@ -123,7 +127,7 @@ impl MultiNetworkModel {
             parameters: Vec::new(),
         });
 
-        idx
+        Ok(idx)
     }
 
     /// Add a transfer of data from one network to another.
@@ -392,4 +396,19 @@ mod tests {
 
         multi_model.step(&mut state).expect("Failed to step multi1-model.")
     }
+
+    #[test]
+    fn test_duplicate_network_names() {
+        let timestepper = default_timestepper();
+        let scenario_collection = ScenarioGroupCollection::default();
+
+        let mut multi_model = MultiNetworkModel::new(ModelDomain::from(timestepper, scenario_collection).unwrap());
+
+        let network = Network::default();
+        let _network1_idx = multi_model.add_network("network1", network);
+        let network = Network::default();
+        let result = multi_model.add_network("network1", network);
+
+        assert!(result.is_err());
+    }
 }