Update error going to 4.2.3

Update error going to 4.2.3

napisao/la Niall Sheehan -
Broj odgovora: 1
We are getting an error regarding a "Duplicate entry '822-29" for mdl_customcert_issues.mdl_custissu_usecus_uix' CREATE UNIQUE INDEX mdl_custissu_usecus_uix ON mdl_cistomcert_issues (userid, customcertid) Error code: ddlexecuteerror

Not sure how to resolve this issue. Please advise.