Do not repeat the same error messages

CHANGELOG.md

@@ -21,6 +21,7 @@ Next Release
 
 #### Fixes
 
+* [#586](https://github.com/intridea/grape/pull/586): Do not repeat the same validation error messages - [@kiela](https://github.com/kiela)
 * [#508](https://github.com/intridea/grape/pull/508): Allow parameters, such as content encoding, in `content_type` - [@dm1try](https://github.com/dm1try).
 * [#492](https://github.com/intridea/grape/pull/492): Don't allow to have nil value when a param is required and has a list of allowed values - [@Antti](https://github.com/Antti).
 * [#495](https://github.com/intridea/grape/pull/495): Fixed `ParamsScope#params` for parameters nested inside arrays - [@asross](https://github.com/asross).

lib/grape/exceptions/validation_errors.rb

@@ -27,7 +27,7 @@ def each
       private
 
       def full_messages
-        map { |attribute, error| full_message(attribute, error) }
+        map { |attribute, error| full_message(attribute, error) }.uniq
       end
 
       def full_message(attribute, error)

spec/grape/exceptions/validation_errors_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+require 'ostruct'
+
+describe Grape::Exceptions::ValidationErrors do
+  let(:validation_message) { "FooBar is invalid" }
+  let(:validation_error) { OpenStruct.new(param: validation_message) }
+
+  context "message" do
+    context "is not repeated" do
+      let(:error) do
+        described_class.new(errors: [validation_error, validation_error])
+      end
+      subject(:message) { error.message.split(',').map(&:strip) }
+
+      it { expect(message).to include validation_message }
+      it { expect(message.size).to eq 1 }
+    end
+  end
+end