diff --git a/lib/http/form_data/multipart.rb b/lib/http/form_data/multipart.rb
index a667643..2724b2b 100644
--- a/lib/http/form_data/multipart.rb
+++ b/lib/http/form_data/multipart.rb
@@ -16,10 +16,8 @@ class Multipart
 
       # @param [#to_h, Hash] data form data key-value Hash
       def initialize(data, boundary: self.class.generate_boundary)
-        parts = Param.coerce FormData.ensure_hash data
-
         @boundary = boundary.to_s.freeze
-        @io = CompositeIO.new [*parts.flat_map { |part| [glue, part] }, tail]
+        @io = CompositeIO.new(parts(data).flat_map { |part| [glue, part] } << tail)
       end
 
       # Generates a string suitable for using as a boundary in multipart form
@@ -54,6 +52,14 @@ def glue
       def tail
         @tail ||= "--#{@boundary}--#{CRLF}"
       end
+
+      def parts(data)
+        if data.is_a?(Array)
+          Param.coerce data
+        else
+          Param.coerce FormData.ensure_hash data
+        end
+      end
     end
   end
 end
diff --git a/lib/http/form_data/multipart/param.rb b/lib/http/form_data/multipart/param.rb
index 5972752..327ad2b 100644
--- a/lib/http/form_data/multipart/param.rb
+++ b/lib/http/form_data/multipart/param.rb
@@ -41,11 +41,11 @@ def initialize(name, value)
           @io = CompositeIO.new [header, @part, footer]
         end
 
-        # Flattens given `data` Hash into an array of `Param`'s.
-        # Nested array are unwinded.
+        # Flattens given `data` Hash or Array into an array of `Param`'s.
+        # Nested arrays are unwinded.
         # Behavior is similar to `URL.encode_www_form`.
         #
-        # @param [Hash] data
+        # @param [Array || Hash] data
         # @return [Array<FormData::MultiPart::Param>]
         def self.coerce(data)
           params = []
diff --git a/spec/lib/http/form_data/multipart_spec.rb b/spec/lib/http/form_data/multipart_spec.rb
index a6d3f55..dc8ee9d 100644
--- a/spec/lib/http/form_data/multipart_spec.rb
+++ b/spec/lib/http/form_data/multipart_spec.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 RSpec.describe HTTP::FormData::Multipart do
-  subject(:form_data) { HTTP::FormData::Multipart.new params }
+  subject(:form_data) { described_class.new params }
 
   let(:file)          { HTTP::FormData::File.new fixture "the-http-gem.info" }
   let(:params)        { { :foo => :bar, :baz => file } }
@@ -17,7 +17,6 @@ def disposition(params)
 
     it "properly generates multipart data" do
       boundary_value = form_data.boundary
-
       expect(form_data.to_s).to eq([
         "--#{boundary_value}#{crlf}",
         "#{disposition 'name' => 'foo'}#{crlf}",
@@ -87,6 +86,45 @@ def disposition(params)
         ].join)
       end
     end
+
+    # https://github.com/httprb/http/issues/663
+    context "when params is an Array of pairs" do
+      let(:params) do
+        [
+          ["metadata", %(filename="first.txt")],
+          ["file", HTTP::FormData::File.new(StringIO.new("uno"), :content_type => "plain/text", :filename => "abc")],
+          ["metadata", %(filename="second.txt")],
+          ["file", HTTP::FormData::File.new(StringIO.new("dos"), :content_type => "plain/text", :filename => "xyz")],
+          ["metadata", %w[question=why question=not]]
+        ]
+      end
+
+      it "allows duplicate param names and preserves given order" do
+        expect(form_data.to_s).to eq([
+          %(--#{form_data.boundary}\r\n),
+          %(Content-Disposition: form-data; name="metadata"\r\n),
+          %(\r\nfilename="first.txt"\r\n),
+          %(--#{form_data.boundary}\r\n),
+          %(Content-Disposition: form-data; name="file"; filename="abc"\r\n),
+          %(Content-Type: plain/text\r\n),
+          %(\r\nuno\r\n),
+          %(--#{form_data.boundary}\r\n),
+          %(Content-Disposition: form-data; name="metadata"\r\n),
+          %(\r\nfilename="second.txt"\r\n),
+          %(--#{form_data.boundary}\r\n),
+          %(Content-Disposition: form-data; name="file"; filename="xyz"\r\n),
+          %(Content-Type: plain/text\r\n),
+          %(\r\ndos\r\n),
+          %(--#{form_data.boundary}\r\n),
+          %(Content-Disposition: form-data; name="metadata"\r\n),
+          %(\r\nquestion=why\r\n),
+          %(--#{form_data.boundary}\r\n),
+          %(Content-Disposition: form-data; name="metadata"\r\n),
+          %(\r\nquestion=not\r\n),
+          %(--#{form_data.boundary}--\r\n)
+        ].join)
+      end
+    end
   end
 
   describe "#size" do
diff --git a/spec/lib/http/form_data/urlencoded_spec.rb b/spec/lib/http/form_data/urlencoded_spec.rb
index be25a38..02d1e21 100644
--- a/spec/lib/http/form_data/urlencoded_spec.rb
+++ b/spec/lib/http/form_data/urlencoded_spec.rb
@@ -4,6 +4,12 @@
   let(:data) { { "foo[bar]" => "test" } }
   subject(:form_data) { HTTP::FormData::Urlencoded.new data }
 
+  it "supports any Enumerables of pairs" do
+    form_data = described_class.new([%w[foo bar], ["foo", %w[baz moo]]])
+
+    expect(form_data.to_s).to eq("foo=bar&foo=baz&foo=moo")
+  end
+
   describe "#content_type" do
     subject { form_data.content_type }
     it { is_expected.to eq "application/x-www-form-urlencoded" }