If the system is updated correctly for FB Inboxer, it shouldn't be duplicated. If for some reason it didn't updated correctly
Please delete all leads or row from facebook_rx_conversion_user_list table.
And run this query :
ALTER TABLE `facebook_rx_conversion_user_list` ADD UNIQUE `Client_unique` (`page_table_id`,`client_thread_id`,`client_id`) USING BTREE;
It should solve the duplicate leads problem for FB Inboxer.