diff --git a/lib/subroutine.rb b/lib/subroutine.rb
index b7ef1bc..78c0ce2 100644
--- a/lib/subroutine.rb
+++ b/lib/subroutine.rb
@@ -14,8 +14,18 @@
 require "subroutine/fields"
 require "subroutine/op"
 
+require "logger"
+
 module Subroutine
 
+  def self.logger
+    @logger
+  end
+
+  def self.logger=(logger)
+    @logger = logger
+  end
+
   # Used by polymorphic association fields to resolve the class name to a ruby class
   def self.constantize_polymorphic_class_name(class_name)
     return @constantize_polymorphic_class_name.call(class_name) if defined?(@constantize_polymorphic_class_name)
@@ -38,6 +48,18 @@ def self.include_defaults_in_params?
     false
   end
 
+  def self.field_redefinition_behavior
+    @field_redefinition_behavior ||= :warn
+  end
+
+  def self.field_redefinition_behavior=(symbol)
+    symbol = symbol.to_sym
+    possible = %i[error warn ignore]
+    raise ArgumentError, "#{symbol} must be one of #{possible.inspect}" unless possible.include?(symbol)
+
+    @field_redefinition_behavior = symbol
+  end
+
   def self.inheritable_field_options=(opts)
     @inheritable_field_options = opts.map(&:to_sym)
   end
diff --git a/lib/subroutine/association_fields.rb b/lib/subroutine/association_fields.rb
index d3fd110..1817a47 100644
--- a/lib/subroutine/association_fields.rb
+++ b/lib/subroutine/association_fields.rb
@@ -66,6 +66,8 @@ def field_with_association(field_name, options = {})
         if options[:type]&.to_sym == :association
           config = ::Subroutine::AssociationFields::Configuration.new(field_name, options)
 
+          field_without_association(config.as, config)
+
           if config.polymorphic?
             field config.foreign_type_method, config.build_foreign_type_field
           else
@@ -77,8 +79,6 @@ def field_with_association(field_name, options = {})
           end
 
           field config.foreign_key_method, config.build_foreign_key_field
-
-          field_without_association(config.as, config)
         else
           field_without_association(field_name, options)
         end
diff --git a/lib/subroutine/association_fields/configuration.rb b/lib/subroutine/association_fields/configuration.rb
index 017e9bb..f534610 100644
--- a/lib/subroutine/association_fields/configuration.rb
+++ b/lib/subroutine/association_fields/configuration.rb
@@ -85,6 +85,7 @@ def build_child_field(name, opts = {})
         child_opts = inheritable_options
         child_opts.merge!(opts)
         child_opts[:association_name] = as
+        child_opts[:allow_override] = true
         ComponentConfiguration.new(name, child_opts)
       end
 
diff --git a/lib/subroutine/fields.rb b/lib/subroutine/fields.rb
index 893ebb4..5b94abd 100644
--- a/lib/subroutine/fields.rb
+++ b/lib/subroutine/fields.rb
@@ -7,6 +7,8 @@
 module Subroutine
   module Fields
 
+    DuplicateFieldError = Class.new(StandardError)
+
     extend ActiveSupport::Concern
 
     def self.allowed_input_classes
@@ -30,6 +32,16 @@ def self.action_controller_params_loaded?
     module ClassMethods
 
       def field(field_name, options = {})
+
+        if field_configurations.key?(field_name.to_sym) && !options[:allow_override]
+          case Subroutine.field_redefinition_behavior
+          when :error
+            raise DuplicateFieldError, "[subroutine] #{self} redefined `#{field_name}`. Add `allow_override: true` to reconfigure the field."
+          when :warn
+            Subroutine.logger&.warn("[subroutine] #{self} redefines `#{field_name}`. Add `allow_override: true` to silence this warning.\nCalled from: #{caller.join("\n")}")
+          end
+        end
+
         config = ::Subroutine::Fields::Configuration.from(field_name, options)
         config.validate!
 
diff --git a/lib/subroutine/version.rb b/lib/subroutine/version.rb
index dfe7176..585ceb6 100644
--- a/lib/subroutine/version.rb
+++ b/lib/subroutine/version.rb
@@ -2,10 +2,10 @@
 
 module Subroutine
 
-  MAJOR = 4
-  MINOR = 5
+  MAJOR = 5
+  MINOR = 0
   PATCH = 0
-  PRE   = nil
+  PRE   = "alpha2"
 
   VERSION = [MAJOR, MINOR, PATCH, PRE].compact.join(".")
 
diff --git a/test/subroutine/fields_test.rb b/test/subroutine/fields_test.rb
index dbde7ec..6869b27 100644
--- a/test/subroutine/fields_test.rb
+++ b/test/subroutine/fields_test.rb
@@ -214,5 +214,32 @@ def test_group_fields_are_not_mutated_by_subclasses
       assert_equal(%i[food], MutationChild.fields_by_group[:four_letter].sort)
     end
 
+    def test_redefinition_of_field__error
+      Subroutine.stubs(:field_redefinition_behavior).returns(:error)
+      Subroutine.stubs(:logger).returns(mock)
+      Subroutine.logger.expects(:warn).never
+      assert_raises Subroutine::Fields::DuplicateFieldError do
+        Whatever.send(:field, :foo, type: :string, default: "foo")
+      end
+    end
+
+    def test_redefinition_of_field__warn
+      Subroutine.stubs(:field_redefinition_behavior).returns(:warn)
+      Subroutine.stubs(:logger).returns(mock)
+      Subroutine.logger.expects(:warn).with(includes("[subroutine] #{Whatever} redefines `foo`. Add `allow_override: true` to silence this warning.")).once
+      Whatever.send(:field, :foo, type: :string, default: "foo")
+    end
+
+    def test_redefinition_of_field__ignore
+      Subroutine.stubs(:field_redefinition_behavior).returns(:ignore)
+      Subroutine.stubs(:logger).returns(mock)
+      Subroutine.logger.expects(:warn).never
+
+      foo_config1 = Whatever.field_configurations[:foo]
+      Whatever.send(:field, :foo, type: :string, default: "foo")
+      foo_config2 = Whatever.field_configurations[:foo]
+      refute_equal foo_config1.object_id, foo_config2.object_id
+    end
+
   end
 end
diff --git a/test/support/ops.rb b/test/support/ops.rb
index f5a2bc4..ff83eb3 100644
--- a/test/support/ops.rb
+++ b/test/support/ops.rb
@@ -154,7 +154,7 @@ class OnlyFooBarOp < ::Subroutine::Op
 
 class InheritedDefaultsOp < ::DefaultsOp
 
-  field :bar, default: "barstool", allow_overwrite: true
+  field :bar, default: "barstool", allow_override: true
 
 end
 
@@ -354,7 +354,7 @@ class SimpleAssociationWithStringIdOp < ::OpWithAssociation
 
 class UnscopedSimpleAssociationOp < ::OpWithAssociation
 
-  association :user, unscoped: true, allow_overwrite: true
+  association :user, unscoped: true, allow_override: true
 
 end