Reject duplicate target+field in override DSL

Two override statements that target the same gem and the same field
make it ambiguous which operation should win. Raise ArgumentError
when the new (target, field) pair already exists in the recorded
overrides, leaving the previously recorded entry untouched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hsbt

bundler/lib/bundler/dsl.rb

@@ -198,6 +198,7 @@ def override(target, **operations)
       operations.each do |field, operation|
         validate_override_field!(field)
         validate_override_operation!(operation)
+        validate_override_uniqueness!(target, field)
       end
 
       operations.each do |field, operation|
@@ -290,6 +291,11 @@ def validate_override_operation!(operation)
       end
     end
 
+    def validate_override_uniqueness!(target, field)
+      return unless @overrides.any? {|o| o.target == target && o.field == field }
+      raise ArgumentError, "duplicate override for #{target.inspect} `#{field}:`"
+    end
+
     def add_dependency(name, version = nil, options = {})
       options["gemfile"] = @gemfile
       options["source"] ||= @source

spec/bundler/dsl_spec.rb

@@ -447,5 +447,27 @@
       end.to raise_error(ArgumentError, /unsupported override field/)
       expect(subject.overrides).to eq([])
     end
+
+    it "raises ArgumentError when the same target and field are overridden twice" do
+      subject.override("rails", version: ">= 8.0")
+      expect do
+        subject.override("rails", version: :ignore_upper)
+      end.to raise_error(ArgumentError, /duplicate override for "rails" `version:`/)
+    end
+
+    it "keeps the original override when a duplicate is rejected" do
+      subject.override("rails", version: ">= 8.0")
+      expect do
+        subject.override("rails", version: :ignore_upper)
+      end.to raise_error(ArgumentError)
+      expect(subject.overrides.size).to eq(1)
+      expect(subject.overrides.first.operation).to eq(">= 8.0")
+    end
+
+    it "allows different targets with the same field" do
+      subject.override("rails", version: ">= 8.0")
+      subject.override("nokogiri", version: :ignore_upper)
+      expect(subject.overrides.size).to eq(2)
+    end
   end
 end