Allison is correct here. I just did a test of two nurture programs. They each contained the same email (unique ID). The first was set to cast at 4PM. The second was set to cast at 4:05PM.
The first nurture program sent the email at 4PM, then marked the lead as exhausted.
The second nurture program did not send the email but marked the lead as exhausted at 4:05PM
Summary: If the same unique email is a part of two different nurture campaigns, any given lead will only receive the email once.