3430868 - "The identity provider was unable to process the authentication request it received" error logging IAS

Login to IAS, you get error:

 "The identity provider cannot process the authentication request it received. 
 Clear your browser's cache and cookies and restart your browser.
If this does not resolve the issue, contact your system administrator.

You can provide the following correlation ID to your administrator: ABCDEFGH-9876-IJKL-5432-MNOPQ109RS87" 

SPLUNK search with correlation ID "ABCDEFGH-9876-IJKL-5432-MNOPQ109RS87" shows:
yyyy-mm-ddThh.mm.ss.xxxZ timestamp="yyyy-mm-ddThh.mm.ss.xxxZ", ipAddress="123.456.789.123", severity="ERROR", location="com.sap.security.saml2.idp.endpoints.sso.SSOEndpointErrorLogger", crtAccount="tenant", authenticatedSubject="anonymous", correlationId="ABCDEFGH-9876-IJKL-5432-MNOPQ109RS87", message="Identity Provider could not process the authentication request sent over Redirect due to general error. Multiple users found with the same identifier which is not allowed in custom tenant. Correlation ID: ABCDEFGH-9876-IJKL-5432-MNOPQ109RS87" 

Identity Authentication Service

"The identity provider cannot process the authentication request it received",
"Identity Provider could not process the authentication request sent over Redirect due to general error",
"Identity Provider could not process the authentication request",
"Multiple users found with the same identifier which is not allowed in custom tenant",
"Multiple users found with the same identifier" , KBA , BC-IAM-IDS , Identity Authentication Service , Problem