If I'm using the same email in two nurtures and a lead goes through both nurtures, will Marketo make sure that they don't receive the same email twice?
Depends on the nurture stream. If you are using a customer engagement Engine it will not send twice. if it is a standard trigger type nurture it will send twice.
Please see docs.marketo.com. There is also a similar post yesterday where I posted help.
Engagements will automatically NOT send the same Email to the same lead as long as it is the same asset ID. This is not easily done using regular smart campaigns/triggers/drips.
What if the lead received the same email in a separate email blast, prior to becoming a member of the engagement program? Would they still receive the email within the nurture? Or, alternatively, if they're already a member of the nurture program and the same email is sent via an email blast, they won't receive it correct?