fix(query): Prevent multiple entries for same predicate in mutations.2022

[dshekhar95]

Problem

Multiple entries for the same predicate can lead to inconsistent state,

Solution

hence adding an error for it instead.

Fixes DGRAPH-3225

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[dshekhar95]

https://github.com/dgraph-io/dgraph/pull/8294

[coveralls]

Coverage increased (+0.07%) to 37.166% when pulling 882d6bf9c0fb9a2158c4d1834d4c1fc85f4be079 on cherry-pick-7715_new2 into 9b670174e6af72b773ae59350d7fa3f75e91c80f on main.