diff --git a/lib/appraisal/gemfile.rb b/lib/appraisal/gemfile.rb
index 94d12ac0..f251acef 100644
--- a/lib/appraisal/gemfile.rb
+++ b/lib/appraisal/gemfile.rb
@@ -97,7 +97,7 @@ def gemspec(options = {})
     protected
 
     def source_entry
-      @sources.map { |source| "source #{source.inspect}" }.join("\n")
+      @sources.uniq.map { |source| "source #{source.inspect}" }.join("\n")
     end
 
     def ruby_version_entry
diff --git a/spec/appraisal/gemfile_spec.rb b/spec/appraisal/gemfile_spec.rb
index f61aef7e..91637063 100644
--- a/spec/appraisal/gemfile_spec.rb
+++ b/spec/appraisal/gemfile_spec.rb
@@ -17,6 +17,13 @@
     expect(gemfile.to_s.strip).to eq %{source "one"\nsource "two"}
   end
 
+  it "ignores duplicate sources" do
+    gemfile = Appraisal::Gemfile.new
+    gemfile.source "one"
+    gemfile.source "one"
+    expect(gemfile.to_s.strip).to eq %{source "one"}
+  end
+
   it "preserves dependency order" do
     gemfile = Appraisal::Gemfile.new
     gemfile.gem "one"