Ignore duplicated `contexts` entries in `script.addPreloadScript`, `session.subscribe` and `session.unsubscribe`

[sadym-chromium]

Currently, if the same context is set twice in the contexts param of the script.addPreloadScript command, the required scripts is added twice to that context. I wonder if this is correct and expected behavior, or we should throw exception / ignore the duplicate?

[whimboo]

Do we actually have the same situation with session.subscribe and session.unsubscribe? I'm fine with ignoring duplicated entries by defining it maybe as a Set?

[sadym-chromium]

I'm fine with set