fix(query): Prevent multiple entries for same predicate in mutations. #7715

[skrdgraph]

we had NOT squashed & merged this #8319

So fixing it up here

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
❌ rohanprasad
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[joshua-goldstein]

👍

[coveralls]

Coverage increased (+0.06%) to 37.165% when pulling 882d6bf on cherry-pick-7715_new2 into a2babb8 on main.