This is on the wiki:
Product type deals can be resubmitted if they expire, if the price goes down, or after 30 days but not before 24 hours pass since the previous submission.
So that means:
1. Product deals can be resubmitted for the same product, price, and store if the original deal expired, even if the new deal is the same.
2. Product deals can be repeated if the price reduces.
3. Product deals can be resubmitted for the same product, price, and store 30 days after the previous posting.
4. Ummm... not sure where this part fits in: "not before 24 hours pass since the previous submission."