Allgemein zu 5.1.4-RC (Im Backend update auf 5.1.4-RC trotz stable)

Tuts bei mir im Testshop nicht…

Error
Received the following error message:
Could not apply migration: SQLSTATE[42000]: Syntax error or access violation: 1061 Duplicate key name 'mapping_id'

Please try to fix this error and restart the update.
Response
{"valid":false,"errorMsg":"Could not apply migration: SQLSTATE[42000]: Syntax error or access violation: 1061 Duplicate key name 'mapping_id'"}