The short answer is no. They won't receive the same email again.
Here's how it was explained to me:
In the case of a stream, Marketo always starts at the top and sees if the lead has received email 1. Then it moves on to email 2, 3, and so on until it finds an email that the lead hasn't received. If the lead has received all emails in the stream, they won't get anything. But you can shuffle the emails any way you want once they've been added to the stream. We recently won an award for an installation we did, and we jumped that email to the top of the stream to ensure that no matter how many emails in the stream our leads had received, THIS would be the next one they got.