Don't follow account if it's already followed (#3575)

Closes https://github.com/tootsuite/mastodon/issues/3102

spec/services/follow_service_spec.rb

@@ -29,6 +29,19 @@ RSpec.describe FollowService do
         expect(sender.following?(bob)).to be true
       end
     end
+
+    describe 'already followed account' do
+      let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob')).account }
+
+      before do
+        sender.follow!(bob)
+        subject.call(sender, bob.acct)
+      end
+
+      it 'keeps a following relation' do
+        expect(sender.following?(bob)).to be true
+      end
+    end
   end
 
   context 'remote account' do
@@ -76,5 +89,26 @@ RSpec.describe FollowService do
         expect(a_request(:post, "http://hub.example.com/")).to have_been_made.once
       end
     end
+
+    describe 'already followed account' do
+      let(:bob) { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob', domain: 'example.com', salmon_url: 'http://salmon.example.com', hub_url: 'http://hub.example.com')).account }
+
+      before do
+        sender.follow!(bob)
+        subject.call(sender, bob.acct)
+      end
+
+      it 'keeps a following relation' do
+        expect(sender.following?(bob)).to be true
+      end
+
+      it 'does not send a follow salmon slap' do
+        expect(a_request(:post, "http://salmon.example.com/")).not_to have_been_made
+      end
+
+      it 'does not subscribe to PuSH' do
+        expect(a_request(:post, "http://hub.example.com/")).not_to have_been_made
+      end
+    end
   end
 end