Raise an Error for Duplicate Hash Keys in Plugin Configs

[original-brownbear]

Follow up to #8597

We want to stop supporting this:

foo => { "bar" => "hello" }
foo => { "baz" => "goodbye" }

which currently gets interpreted as:

foo => {
  "bar" => "hello"
  "baz" => "goodbye"
}

and instead, want to raise a config error when encountering a duplicate key in a plugin config.

Also as @jordansissel mentions below:

[...] we should also add this to 6.x but instead of an error (as in 7.0) it logs a warning w/ how to fix the issue.

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Note that this is supposed to only be merged to 7.x (master as of the time of writing) not 6.x.

[jordansissel]

@original-brownbear we should also add this to 6.x but instead of an error (as in 7.0) it logs a warning w/ how to fix the issue.

[original-brownbear]

@jordansissel thanks, added it to the issue :)