Adrien Grand 5d5c6591aa Validate that fields are defined only once.
There are two ways that a field can be defined twice:
 - by reusing the name of a meta mapper in the root object (`_id`, `_routing`,
   etc.)
 - by defining a sub-field both explicitly in the mapping and through the code
   in a field mapper (like ExternalMapper does)

This commit adds new checks in order to make sure this never happens.

Close #15057
2015-12-15 10:37:18 +01:00
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