It seems that the relation validations are not covering all the cases and if you try to add two duplicated follows relations, the request fails with 500 internal error. The issue reproduces only on 4.2.* versions and it's caused by #33418.

I, [2021-08-01T16:29:31.426087 #19530]  INFO -- : Completed 500 Internal Server Error in 117ms (ActiveRecord: 70.3ms | Allocations: 8146)


F, [2021-08-01T16:29:31.427135 #19530] FATAL -- :   
ActiveRecord::RecordNotUnique (Mysql2::Error: Duplicate entry '1-2' for key 'issue_relations.index_issue_relations_on_issue_from_id_and_issue_to_id'):
  
app/controllers/issue_relations_controller.rb:67:in `block in create'
app/controllers/issue_relations_controller.rb:51:in `each'
app/controllers/issue_relations_controller.rb:51:in `create'