vreplication: error out if source table is mentioned twice in Materialize

[derekperkins]

If you reference the same source table twice in the same Materialize workflow (and presumably other streams, though untested), the second reference doesn't get populated, but transitions into RUNNING without throwing an error. On Slack, @sougou confirmed that this is how it functions.

it's very likely that vreplication assumes that the source tables are distinct (per stream), yeah. the copy_state supports only one unique table per stream
https://vitess.slack.com/archives/CMKTCUYNQ/p1631687493154700

I'm guessing it's a deeper issue to fix to allow an arbitrary number of references to a source table, which would be the preferred solution. In the meantime, we should check for it and throw an error before executing so that people aren't getting incorrectly materialized data.

Example materialize statement

vtctlclient -server localhost:15999 Materialize '{
  "workflow": "billing__usage__vrepl",
  "source_keyspace": "workspaces",
  "target_keyspace": "workspaces",
  "table_settings": [
    {
      "target_table": "billing__usage__vrepl",
      "source_expression": "select workspace_id as account_id, date(from_unixtime(requested)) as requested_date, date(created_at) as created_date, 2 as type, count(*) as usage from workspaces_rankings__pulls where is_priority=1 group by account_id, requested_date, created_date, type"
    },
    {
      "target_table": "billing__usage__vrepl",
      "source_expression": "select workspace_id as account_id, date(from_unixtime(requested)) as requested_date, date(created_at) as created_date, 1 as type, count(*) as usage from workspaces_rankings__pulls where is_priority=0 group by account_id, requested_date, created_date, type"
    }
  ],
  "tablet_types": "MASTER"
}'

Binary version

11.0.0