Dual representation is when one agent acts for both the player and a club in the same deal, and is paid by both. It is permitted under disclosure rules but creates an obvious conflict of interest, because the same person is negotiating against themselves on fee and wages.
The conflict is structural. An agent representing the player wants the highest wage and the best terms; the same agent representing the buying club is meant to be helping it pay less. Both sides are paying the same person to argue against the other one.
Regulators have generally handled it through consent and disclosure rather than a ban, requiring everyone involved to agree in writing that they know it is happening. FIFA's agent regulations tightened the rules around who may be paid by whom, and how much of that must be declared.
It is one reason the published agent payment figures are worth reading. They do not separate a straightforward commission from a fee earned on both sides of the same transaction, but the totals are the only routine window the public gets into how the money moves.