Last tested: May 19, 2020
When "Mirror SAML Groups" is toggled on in your instance's SAML settings, it will create a new group in Looker based off of the IdP-side group you input.
A question was asked, "if the IdP side group has the same name as an existing Looker group, will saving those settings merge the two groups together in Looker?"
No, SAML-mapped groups are treated as a separate thing and if the group name you save when "Mirror SAML Groups" is toggled on matches the name of an existing Looker group you will see the below error when trying to save your SAML settings.
group named <name of group> already exists. 'saml_group'.