diff --git a/lib/spack/spack/mixins.py b/lib/spack/spack/mixins.py
index d757a963b3..2ce80c4c39 100644
--- a/lib/spack/spack/mixins.py
+++ b/lib/spack/spack/mixins.py
@@ -118,6 +118,9 @@ def __init__(cls, name, bases, attr_dict):
             for f in fn_list:
                 phase_obj.run_before.append(f)
 
+        # Flush the dictionary for the next class
+        PackageMixinsMeta._add_method_before.clear()
+
         for phase in PackageMixinsMeta._add_method_after:
 
             attr_name = attr_fmt.format(phase)
@@ -127,6 +130,9 @@ def __init__(cls, name, bases, attr_dict):
             for f in fn_list:
                 phase_obj.run_after.append(f)
 
+        # Flush the dictionary for the next class
+        PackageMixinsMeta._add_method_after.clear()
+
         super(PackageMixinsMeta, cls).__init__(name, bases, attr_dict)