PERF: use labels to find duplicates in multi-index

    -------------------------------------------------------------------------------
    Test name                                    | head[ms] | base[ms] |  ratio   |
    -------------------------------------------------------------------------------
    multiindex_duplicated                        | 139.2883 | 469.6666 |   0.2966 |
    -------------------------------------------------------------------------------
    Test name                                    | head[ms] | base[ms] |  ratio   |
    -------------------------------------------------------------------------------

    Ratio < 1.0 means the target commit is faster then the baseline.
    Seed used: 1234

    Target [2b99b80] : use labels to identify duplicates in multi-index
    Base   [0fe43a6] : Merge pull request #9120 from minrk/nbviewer-link

    fix an nbviewer link in tutorials