feat(sync): Handle duplicate clients when multiplexing

[erezrokah]

Related to cloudquery/cloudquery#8099 and other PK issues

Due to configuration errors, bugs, etc., multiplexers can generate duplicate clients, that is more than one client with the same fields to use during sync.

We should detect that here

plugin-sdk/plugins/source/scheduler_dfs.go

Line 38 in 963f03c

clients = table.Multiplex(client)

(using client.ID()), skip any duplicate clients and add a log warning for those (possibly send it to sentry too).

[erezrokah]

Closing as phase 1 done in #723 (phase 2 might be to skip those duplicated based on data from phase 1)