Also, in case this needs restating, you can't change email addresses to plus addresses, even in those cases where they "work" for splitting up outbound email, as the lead will never identify him/herself this way in the future.
HI
Create a smart campaign that is a triggered one, using the "campaign is requested" trigger.
Then create your batch that includes all the contacts in the account, and in this batch, use the "request campaign" flow step to call the triggered campaign above. As the campaign is triggered, it will not dedupe on email before sending .
-Greg
Thanks every one for your comments! I will try the trigger solution and see if that works.
The solution that Grégoire Michel mentioned was actually presented in Melbourne Marketo User Group by David Kreitter <dkreitter@marketo.com> on Tuesday as he had encountered companies who want to send to same email twice. Example he gave was a. couple who use same home email address and company want to send same email to both Susan Doe and John Doe and hence one email is sent Dear Susan ... and the other is Dear John ....