In campaign builder, you may choose between 2 types of emails :
Transactional emails :
It can be sent multiple times to the same contact accross campaigns.
Marketing emails :
It can be sent only once to the same contact.
A contact received "email 1" from campaign #1.
This contact just got into the campaign #2 et should receive "email 1". If this email is in marketing type, it won't be sent to this contact as he already received it in campaign #1.