Add protection for duplicate index when cloning MCVertex

As discussed in LHCBGAUSS-1065 - See https://its.cern.ch/jira/browse/LHCBGAUSS-1065?focusedCommentId=1663175&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-1663175