topo2topo copy of shard replication is not idempotent

[zhongr3n]

Overview of the Issue

While using topo2topo to migrate topology data, we noticed that while the copying of all other topology data are idempotent, the copying of shard replication is not. Instead, it appends the tablets to the ShardReplication struct every time copy is run.

The code is doing a protobuf merge, which is probably simply appending:
https://github.com/vitessio/vitess/blob/main/go/vt/topo/helpers/copy.go#L172

Reproduction Steps

1. Follow this guide to copy topology data , but run topo2topo -do-shard-replications twice
2. Run topo2topo -do-shard-replications -compare and the validation will fail.
3. Use topology specified CLI tool (e.g. etcdcli) to view the shard replication data

Binary Version

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Operating System and Environment details

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Log Fragments

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