4.7.20 upgrade -- missing indices warning, but index already exists?
This thread helped me:

How to find and/or fix missing indices?

I've aggregated the SQL listed there below, copy it and run it via phpMyAdmin:

ALTER TABLE civicrm_prevnext_cache DROP INDEX index_all;

ALTER TABLE civicrm_option_value DROP FOREIGN KEY FK_civicrm_option_value_option_group_id;

ALTER TABLE civicrm_option_value DROP INDEX index_option_group_id_name;

ALTER TABLE civicrm_entity_tag DROP INDEX UI_entity_id_entity_table_tag_id; 

ALTER TABLE civicrm_case_contact DROP FOREIGN KEY FK_civicrm_case_contact_contact_id; 

ALTER TABLE civicrm_case_contact DROP FOREIGN KEY FK_civicrm_case_contact_case_id; 

ALTER TABLE civicrm_case_contact DROP INDEX UI_case_contact_id;


then click on the Update Indices button.   Then go back to phpMyAdmin and run these SQL commands:

ALTER TABLE civicrm_option_value ADD CONSTRAINT `FK_civicrm_option_value_option_group_id` FOREIGN KEY (`option_group_id`) REFERENCES `civicrm_option_group` (`id`) ON DELETE CASCADE;

ALTER TABLE civicrm_case_contact ADD CONSTRAINT FK_civicrm_case_contact_contact_id FOREIGN KEY (contact_id) REFERENCES civicrm_contact (id) ON DELETE CASCADE;

ALTER TABLE civicrm_case_contact ADD CONSTRAINT FK_civicrm_case_contact_case_id FOREIGN KEY (case_id) REFERENCES civicrm_case (id) ON DELETE CASCADE;


I had this issue moving from 4.7.18 to 4.7.20.   Not sure why this wasn't an issue before, I'm guessing its because I don't use the CASE module on my site.