WHAT IS THE DIFFERENCE BETWEEN DUAL AGENCY AND DESIGNATED AGENCY IN A VIRGINIA REAL ESTATE TRANSACTION?

There are different forms of representation to you that include: 
Buyer Agency or Selling Agency (when an Agent represents a Buyer) or
Listing Agency or
Seller Agency - NOT TO BE CONFUSED WITH Selling Agency.
Selling Agency (when an Agent represents the Seller) or Non-Agency (when the Party in question is not represented by an Agent at all).
Agency Representation for For Sale By Owner Home Sellers Flat Fee MLS Listings FSBOs
When you hire me to help you buy or sell real estate, you are really hiring my brokerage, Cottage Street Realty.
I am merely the designated associate to help you through your transaction. Cottage Street Realty has many licensed real estate agents working for the their brokerage.

If you hired me as your Cottage Street Realty representative to sell your home, another Cotttage Street Realty agent bringing a buyer would be a form of DESIGNATED REPRESENTATION.
The brokerage, after all, is handling both sides of the transaction, but each party has their own representative.
If you hired me to help you purchase a home, and you wanted to buy a home listed by another Cottage Street Realty agent, that would also result in designated representation.

DUAL REPRESENTATION would occur if you hired me to sell your home and I also represented the buyers that wanted to buy it.
You would also run into dual representation if you hired me to help you purchase a home and you wanted to buy one of MY listings.

Dual representation means I cannot advise either party to things like price so some sellers and buyers do not like this and some don't care.
So if you do have a problem with this, I can always refer you to a very competent professional in our company that will be able to actually represent you.