It seems not:

Given:

    function Fred {
        begin {
            echo "hello from begin1"
        }
        begin {
            echo "hello from begin2"
        }
        process {
            echo "does the magic"
        }
    }
    $bob = @("alpha" "beta")
    $bob | Fred

    $ pwsh fred.ps1
    ParserError: /Users/mdaniel/fred.ps1:5
    Line |
       5 |      begin {
         |      ~~~~~~~
         | Script command clause 'begin' has already been defined.