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

[rohanprasad]

Multiple entries for the same predicate can lead to inconsistent state, hence adding an error for it instead.

Fixes DGRAPH-3225

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This change is 

[ahsanbarkati]

Awesome @rohanprasad. I would want similar check for types as well.

<name>: string .
<age>: int .
type <Person> {
  name
}
type <Person> {
  age
}

The above schema update should also complain. Currently we accept this schema.

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @pawanrawal)

[NamanJain8]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @pawanrawal and @rohanprasad)

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query/mutation.go, line 39 at r1 (raw file):

// isValidSchemaChange check if the mutation has valid schema changes. In case the mutation
// has multiple entries for the same predicate we error out.
func isValidSchemaChange(m *pb.Mutations) error {

Looks good. Just a comment.
Most of the cases for schema update are handled in parseSchemaFromAlterOperation. I think it would be better to make these checks there.

[rohanprasad]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @pawanrawal and @rohanprasad)

query/mutation.go, line 39 at r1 (raw file):

// isValidSchemaChange check if the mutation has valid schema changes. In case the mutation
// has multiple entries for the same predicate we error out.
func isValidSchemaChange(m *pb.Mutations) error {

Looks good. Just a comment.
Most of the cases for schema update are handled in parseSchemaFromAlterOperation. I think it would be better to make these checks there.

Done

[NamanJain8]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @manishrjain, @NamanJain8, @pawanrawal, and @vvbalaji-dgraph)

Done