diff --git a/src/classy_blocks/modify/junction.py b/src/classy_blocks/modify/junction.py
index 6417487..8660f61 100644
--- a/src/classy_blocks/modify/junction.py
+++ b/src/classy_blocks/modify/junction.py
@@ -5,6 +5,10 @@
 from classy_blocks.modify.clamps.clamp import ClampBase
 
 
+class ClampExistsError(Exception):
+    """Raised when adding a clamp to a junction that already has one defined"""
+
+
 class Junction:
     """A class that collects Cells/Blocks that
     share the same Vertex"""
@@ -24,6 +28,9 @@ def add_cell(self, cell: Cell) -> None:
                 return
 
     def add_clamp(self, clamp: ClampBase) -> None:
+        if self.clamp is not None:
+            raise ClampExistsError(f"Clamp already defined for junctoin {self.vertex.index}")
+
         self.clamp = clamp
 
     @property
diff --git a/tests/test_optimize/test_optimizer.py b/tests/test_optimize/test_optimizer.py
index c016c90..fd6c21c 100644
--- a/tests/test_optimize/test_optimizer.py
+++ b/tests/test_optimize/test_optimizer.py
@@ -8,6 +8,7 @@
 from classy_blocks.modify.clamps.links import TranslationLink
 from classy_blocks.modify.find.geometric import GeometricFinder
 from classy_blocks.modify.iteration import IterationDriver
+from classy_blocks.modify.junction import ClampExistsError
 from classy_blocks.modify.optimizer import Optimizer
 from classy_blocks.util import functions as f
 from classy_blocks.util.constants import VBIG
@@ -132,6 +133,12 @@ def setUp(self):
 
         self.vertex = next(iter(self.finder.find_in_sphere([0, 0, 0])))
 
+    def test_add_junction_existing(self):
+        self.optimizer.release_vertex(FreeClamp(self.mesh.vertices[0]))
+
+        with self.assertRaises(ClampExistsError):
+            self.optimizer.release_vertex(FreeClamp(self.mesh.vertices[0]))
+
     def test_optimize(self):
         # move a point, then optimize it back to
         # its initial-ish position