diff --git a/lib/thor/actions/inject_into_file.rb b/lib/thor/actions/inject_into_file.rb
index 6062e5a8..4f7791da 100644
--- a/lib/thor/actions/inject_into_file.rb
+++ b/lib/thor/actions/inject_into_file.rb
@@ -108,7 +108,10 @@ def say_status(behavior, warning: nil, color: nil)
       def replace!(regexp, string, force)
         return if pretend?
         content = File.read(destination)
-        if force || !content.include?(replacement)
+        before, after = content.split(regexp, 2)
+        snippet = (behavior == :after ? after : before).to_s
+
+        if force || !snippet.include?(replacement)
           success = content.gsub!(regexp, string)
 
           File.open(destination, "wb") { |file| file.write(content) }
diff --git a/spec/actions/inject_into_file_spec.rb b/spec/actions/inject_into_file_spec.rb
index eca16fb9..36b8d7b5 100644
--- a/spec/actions/inject_into_file_spec.rb
+++ b/spec/actions/inject_into_file_spec.rb
@@ -34,11 +34,21 @@ def file
       expect(File.read(file)).to eq("__start__\nmore content\nREADME\n__end__\n")
     end
 
+    it "ignores duplicates before the after flag" do
+      invoke! "doc/README", "\nREADME", :after => "README"
+      expect(File.read(file)).to eq("__start__\nREADME\nREADME\n__end__\n")
+    end
+
     it "changes the file adding content before the flag" do
       invoke! "doc/README", "more content\n", :before => "__end__"
       expect(File.read(file)).to eq("__start__\nREADME\nmore content\n__end__\n")
     end
 
+    it "ignores duplicates before the before flag" do
+      invoke! "doc/README", "README\n", :before => "README"
+      expect(File.read(file)).to eq("__start__\nREADME\nREADME\n__end__\n")
+    end
+
     it "appends content to the file if before and after arguments not provided" do
       invoke!("doc/README", "more content\n")
       expect(File.read(file)).to eq("__start__\nREADME\n__end__\nmore content\n")