SQL Server 2017 CU21 apply patch error

[gabster12]

Expected behavior and actual behavior:
SQL Server 2017 CU 21 apply patch from sql server 2017 CU20 fails with error: The operation failed because an index or statistics with name 'IX_EventMessageContext_EventMessageId' already exists on table 'internal.event_message_context'.

Steps to reproduce the problem:
Apply patch SQLServer2017-KB4557397-x64.exe over sql server 2017 Developer CU20

Versions:
Microsoft SQL Server 2017 (RTM-CU20) (KB4541283) - 14.0.3294.2 (X64) Mar 13 2020 14:53:45 Copyright (C) 2017 Microsoft Corporation Developer Edition (64-bit) on Windows Server 2019 Standard 10.0 (Build 17763: ) (Hypervisor)