Fix already-followed accounts in follow imports contributing to max follows limit

Fixes #28232

app/models/form/import.rb

@@ -110,7 +110,7 @@ class Form::Import
       when 'Languages'
         field&.split(',')&.map(&:strip)&.presence
       when 'Account address'
-        field.strip.gsub(/\A@/, '')
+        field.strip.delete_prefix('@')
       when '#domain', '#uri', 'List name'
         field.strip
       else
@@ -151,14 +151,29 @@ class Form::Import
     errors.add(:data, I18n.t('imports.errors.over_rows_processing_limit', count: ROWS_PROCESSING_LIMIT)) if csv_row_count > ROWS_PROCESSING_LIMIT
 
     if type.to_sym == :following
-      base_limit = FollowLimitValidator.limit_for_account(current_account)
-      limit = base_limit
-      limit -= current_account.following_count unless overwrite
-      errors.add(:data, I18n.t('users.follow_limit_reached', limit: base_limit)) if csv_row_count > limit
+      follows_limit = FollowLimitValidator.limit_for_account(current_account)
+      errors.add(:data, I18n.t('users.follow_limit_reached', limit: follows_limit)) if follows_count_after_import > follows_limit
     end
   rescue CSV::MalformedCSVError => e
     errors.add(:data, I18n.t('imports.errors.invalid_csv_file', error: e.message))
   rescue EmptyFileError
     errors.add(:data, I18n.t('imports.errors.empty'))
   end
+
+  def follows_count_after_import
+    return csv_row_count if overwrite
+
+    accts = parsed_rows.pluck('acct').compact.map(&:downcase).uniq.map do |acct|
+      username, domain = acct.split('@')
+      if domain.nil? || TagManager.instance.local_domain?(domain)
+        username.downcase
+      else
+        "#{username.downcase}@#{TagManager.instance.normalize_domain(domain)}"
+      end
+    end
+
+    current_follows = current_account.following.select(:username, :domain).map { |account| account.acct.downcase }
+
+    (current_follows + accts).uniq.size
+  end
 end

spec/models/form/import_spec.rb

@@ -67,17 +67,33 @@ RSpec.describe Form::Import do
       end
     end
 
-    context 'when importing more follows than allowed' do
+    context 'when the combination of existing follows and imported follows goes above the limit' do
       let(:import_type) { 'following' }
       let(:import_file) { 'imports.txt' }
 
       before do
-        allow(FollowLimitValidator).to receive(:limit_for_account).with(account).and_return(1)
+        account.follow!(Fabricate(:account))
+        allow(FollowLimitValidator).to receive(:limit_for_account).with(account).and_return(2)
       end
 
       it 'has errors' do
         subject.validate
-        expect(subject.errors[:data]).to include(I18n.t('users.follow_limit_reached', limit: 1))
+        expect(subject.errors[:data]).to include(I18n.t('users.follow_limit_reached', limit: 2))
+      end
+    end
+
+    context 'when the combination of existing follows and imported follows falls within the limit' do
+      let(:import_type) { 'following' }
+      let(:import_file) { 'imports.txt' }
+
+      before do
+        account.follow!(Fabricate(:account, username: 'user', domain: 'example.com'))
+        allow(FollowLimitValidator).to receive(:limit_for_account).with(account).and_return(2)
+      end
+
+      it 'is valid' do
+        subject.validate
+        expect(subject).to be_valid
       end
     end