Don't add duplicated specs to unresolved specs

This could happen when a regular gem shadows a default gem.

lib/rubygems/specification.rb

@@ -1412,7 +1412,7 @@ def activate_dependencies
       end
 
       begin
-        specs = spec_dep.to_specs
+        specs = spec_dep.to_specs.uniq(&:full_name)
       rescue Gem::MissingSpecError => e
         raise Gem::MissingSpecError.new(e.name, e.requirement, "at: #{spec_file}")
       end

test/rubygems/test_require.rb

@@ -182,6 +182,22 @@ def test_require_is_not_lazy_with_exact_req
     assert_equal %w[a-1 b-1], loaded_spec_names
   end
 
+  def test_require_is_not_lazy_with_shadowed_default_gem
+    b1_default = new_default_spec("b", "1", nil, "foo.rb")
+    install_default_gems b1_default
+
+    a1 = util_spec "a", "1", { "b" => ">= 1" }, "lib/test_gem_require_a.rb"
+    b1 = util_spec("b", "1", nil, "lib/foo.rb")
+    install_specs b1, a1
+
+    # Load default ruby gems fresh as if we've just started a ruby script.
+    Gem::Specification.reset
+
+    assert_require "test_gem_require_a"
+    assert_equal %w[a-1 b-1], loaded_spec_names
+    assert_equal unresolved_names, []
+  end
+
   def test_require_is_lazy_with_inexact_req
     a1 = util_spec "a", "1", { "b" => ">= 1" }, "lib/test_gem_require_a.rb"
     b1 = util_spec "b", "1", nil, "lib/b/c.rb"