[ML] Opening a job with duplicate detectors fail

[elasticmachine]

Original comment by @davidkyle:

Create a job with duplicate detectors e.g.

{
    "analysis_config" : {
        "detectors" :[{"function":"count", "partition_field_name":"user"},
                               {"function":"count", "partition_field_name":"user"}]
    },
 ...
}'

The job will fail when you try to open with the reason hidden in the log file

[2018-02-21T14:47:37,308][ERROR][o.e.x.m.j.p.a.NativeAutodetectProcess] [duplicate-job] autodetect process stopped unexpectedly: Token 1 in the analysis clause is a duplicate of token 0
Error reading config file /var/folders/n0/rwqy9jyx54zgbv44d8_srk580000gn/T/elasticsearch.XsfOu4LI/fieldconfig8834784943083392311.conf
Field config could not be interpreted

The validation in x-pack should be the same as in autodetect so this config is rejected at creation.

[sophiec20]

Closing as duplicate, in favour of #39704