diff --git a/lib/deep_preloader/polymorphic_spec.rb b/lib/deep_preloader/polymorphic_spec.rb
index 1918d89..ab21cba 100644
--- a/lib/deep_preloader/polymorphic_spec.rb
+++ b/lib/deep_preloader/polymorphic_spec.rb
@@ -35,7 +35,7 @@ def merge!(other)
         if specs_by_type[k]
           specs_by_type[k].merge!(v)
         else
-          specs_by_type[k] = v
+          specs_by_type[k] = v.deep_dup
         end
       end
     else
@@ -54,6 +54,10 @@ def ==(other)
 
   alias eql? ==
 
+  def deep_dup
+    self.class.new(specs_by_type.deep_dup)
+  end
+
   def inspect
     "PolySpec#{specs_by_type.inspect}"
   end
diff --git a/lib/deep_preloader/spec.rb b/lib/deep_preloader/spec.rb
index 3a1f738..bab503f 100644
--- a/lib/deep_preloader/spec.rb
+++ b/lib/deep_preloader/spec.rb
@@ -38,7 +38,7 @@ def merge!(other)
         if association_specs[k]
           association_specs[k].merge!(v)
         else
-          association_specs[k] = v
+          association_specs[k] = v.deep_dup
         end
       end
     else
@@ -57,6 +57,10 @@ def ==(other)
 
   alias eql? ==
 
+  def deep_dup
+    self.class.new(association_specs.deep_dup)
+  end
+
   def inspect
     "Spec#{association_specs.inspect}"
   end
diff --git a/spec/unit/deep_preloader_spec.rb b/spec/unit/deep_preloader_spec.rb
index bc69c5d..3b69661 100644
--- a/spec/unit/deep_preloader_spec.rb
+++ b/spec/unit/deep_preloader_spec.rb
@@ -34,6 +34,18 @@
     expect(DeepPreloader::Spec.parse(spec)).to eq(expected_spec)
   end
 
+  it 'can merge specs' do
+    a = DeepPreloader::Spec.new(a: :b)
+    b = DeepPreloader::Spec.new(a: :c)
+    result = DeepPreloader::Spec.new.merge!(a).merge!(b)
+
+    # The result should contain each of the merged specs and the merged specs
+    # themselves should not be altered
+    expect(result).to eq(DeepPreloader::Spec.new(a: [:b, :c]))
+    expect(a).to eq(DeepPreloader::Spec.new(a: :b))
+    expect(b).to eq(DeepPreloader::Spec.new(a: :d))
+  end
+
   context 'with a test model' do
     with_temporary_table(:model)