Cherry-pick #13914 to 7.4: Field registry: Don't overwrite fields with same name#14037

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adriansr merged 1 commit intoelastic:7.4from adriansr:backport_13914_7.4

Oct 14, 2019

Commits on Oct 14, 2019

Field registry: Don't overwrite fields with same name (elastic#13914 )
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