Adding scope for account_deletion to get only those that are uncompleted and then adding that scope to accounts.rake. Tested it out and it works well

app/models/account_deletion.rb

@@ -5,6 +5,7 @@
 class AccountDeletion < ActiveRecord::Base
   include Diaspora::Federated::Base
 
+  scope :uncompleted, -> { where('completed_at is null') }
 
   belongs_to :person
   after_commit :queue_delete_account, :on => :create

lib/tasks/accounts.rake

@@ -1,9 +1,9 @@
 namespace :accounts do
   desc "Run deletions"
   task :run_deletions => :environment do
-    if ::AccountDeletion.count > 0
+    if ::AccountDeletion.uncompleted.count > 0
       puts "Running account deletions.."
-      ::AccountDeletion.find_each do |account_delete|
+      ::AccountDeletion.uncompleted.find_each do |account_delete|
         account_delete.perform!
       end
       puts "OK."