4.3.2 Support for Same Name Siblings is Optional

As mentioned, whether or not a particular node allows multiple child items with the same name is governed by the node type of that particular node. See 6.7 Node Types.

Though there is a required set of node types that every compliant repository must support, none of these required node types allow same-name siblings and any further node types available in a particular repository are implementation-specific. Therefore, it is possible for a repository to disallow same-name siblings altogether by restricting the set of available node types.