Odoo Error: Kindly help this is a weird error i am getting for the first time. I am trying to validate the invoice and it gives me this error. Error, a partner cannot follow twice the same object.

I google out but couldnt get a perfect solution. I followed this link and did the same but still facing the same error.

https://www.odoo.com/forum/help-1/question/constraint-mail-followers-error-cannot-follow-twice-the-same-object-122881

Then I checked the error log:

2018-11-14 16:00:23,911 13291 ERROR syed_look_test_issue odoo.sql_db: bad query: INSERT INTO "mail_followers" ("id", "res_model", "res_id", "partner_id") VALUES(nextval('mail_followers_id_seq'), 'account.invoice', 29, 13) RETURNING id

ERROR: duplicate key value violates unique constraint "mail_followers_mail_followers_res_partner_res_model_id_uniq"

DETAIL:  Key (res_model, res_id)=(account.invoice, 29) already exists.

I have deleted the records containing account.invoice from mail_followers. why still this error exists??

Also i tried this https://github.com/odoo/odoo/issues/15589

But no use... kindly help if someone knows a perfect solution. I using version 11.