Users may need to create a copy of an existing record. For that use case, use the <CloneButton> component. It reads the record from the current RecordContext.

For instance, to allow cloning all the posts from the list:

import * as React from "react";
import { List, Datagrid, TextField, CloneButton } from 'react-admin';

const PostList = () => (
            <TextField source="title" />
            <CloneButton />

Note: <CloneButton> is designed to be used in a <Datagrid> and in an edit view <Actions> component, not inside the form <Toolbar>. The Toolbar is basically for submitting the form, not for going to another resource.