BURs: don't allow requesting implications that already exist.

Fix it being possible to request duplicate implications.

test/unit/bulk_update_request_test.rb

@@ -129,6 +129,14 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
           assert_equal(["Can't create implication a -> c (a already implies c through another implication)"], @bur.errors.full_messages)
         end
 
+        should "fail for an implication that is a duplicate of an existing implication" do
+          create(:tag_implication, antecedent_name: "a", consequent_name: "b")
+          @bur = build(:bulk_update_request, script: "imply a -> b")
+
+          assert_equal(false, @bur.valid?)
+          assert_equal(["Can't create implication a -> b (Implication already exists)"], @bur.errors.full_messages)
+        end
+
         should_eventually "fail for an implication that is redundant with another implication in the same BUR" do
           create(:tag_implication, antecedent_name: "b", consequent_name: "c")
           @bur = build(:bulk_update_request, script: "imply a -> b\nimply a -> c")

test/unit/tag_implication_test.rb

@@ -47,7 +47,7 @@ class TagImplicationTest < ActiveSupport::TestCase
         [ti1, ti2, ti3, ti4, ti5].each { |ti| assert(ti.valid?) }
 
         ti5.update(status: "active")
-        assert_includes(ti5.errors[:antecedent_name], "has already been taken")
+        assert_includes(ti5.errors[:antecedent_name], "Implication already exists")
       end
     end
 
@@ -115,7 +115,7 @@ class TagImplicationTest < ActiveSupport::TestCase
       ti2 = FactoryBot.build(:tag_implication, :antecedent_name => "aaa", :consequent_name => "bbb")
       ti2.save
       assert(ti2.errors.any?, "Tag implication should not have validated.")
-      assert_includes(ti2.errors.full_messages, "Antecedent name has already been taken")
+      assert_includes(ti2.errors.full_messages, "Implication already exists")
     end
 
     should "not validate if its antecedent or consequent are aliased to another tag" do