diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index e34f2b799d3..6cf80754a1d 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -1388,8 +1388,9 @@ def _replace_with(self, concrete):
             dependent = dep_spec.parent
             deptypes = dep_spec.deptypes
 
-            # remove self from all dependents.
-            del dependent._dependencies[self.name]
+            # remove self from all dependents, unless it is already removed
+            if self.name in dependent._dependencies:
+                del dependent._dependencies[self.name]
 
             # add the replacement, unless it is already a dep of dependent.
             if concrete.name not in dependent._dependencies: