propose key same cause assert panic[Found existing proposal with key: [7d02e530d09]]

[yangxuanjia]

2021/02/02 13:09:51 Found existing proposal with key: [7d02e530d09]
github.com/dgraph-io/dgraph/x.AssertTruef
        /export/working/src/github.com/dgraph-io/dgraph/x/error.go:107
github.com/dgraph-io/dgraph/worker.(*node).proposeAndWait.func3
        /export/working/src/github.com/dgraph-io/dgraph/worker/proposal.go:221
github.com/dgraph-io/dgraph/worker.(*node).proposeAndWait.func4
        /export/working/src/github.com/dgraph-io/dgraph/worker/proposal.go:289
github.com/dgraph-io/dgraph/worker.(*node).proposeAndWait
        /export/working/src/github.com/dgraph-io/dgraph/worker/proposal.go:293
github.com/dgraph-io/dgraph/worker.(*grpcWorker).proposeAndWait
        /export/working/src/github.com/dgraph-io/dgraph/worker/mutation.go:814
github.com/dgraph-io/dgraph/worker.proposeOrSend
        /export/working/src/github.com/dgraph-io/dgraph/worker/mutation.go:538
runtime.goexit
        /export/go/src/runtime/asm_amd64.s:1373
[Decoder]: Using assembly version of decoder
Page Size: 4096

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This change is 

[NamanJain8]

Thanks, @yangxuanjia , looks good to me. Dgraph should not crash in random number collision.

[NamanJain8]

This is not needed after this change #7741 as we are generating better non-random numbers (slightly randomized by initUniqueKey). Thanks @yangxuanjia.