People talk to one another

The airline sends a note to both travel agencies and asks them to remove the dupe reservation. In this case you should receive two phone calls and you can decide, which reservation to keep.

But some airlines try to resolve the problem with technical tools:

This means, if they receive a dupe reservation, they do not create in their system a second reservation. Instead they create the second reservation as a link to the orignial one. The travel agent creating the second reservation is not aware of that, and the travel agent with the first reservation is not aware that somebody else is linked to his reservation.

Reservation becomes a link to nowhere

Travel agent number one cancelles your reservation in his system as he becomes aware that you are not going to buy the ticket through him. The reservation of travel agent number two, which was in reality a link to a reservation, is now a link to nowhere. But the travel agent number two is not aware of any problem, and his system shows him still an ok behind your seats.
If the plane is not full, you won’t note anything, because the airline or the travel agent will just restore the reservation, and you get your tickets. But if the plane is full, or the actual price for a seat on the plane is in the meantime much higher than it was before, you will get tickets only for the new price. The difference can be quite big.
The airline and the travel agent will just point to their terms of service, which include the standard clause:

A fare is only guaranteed after a ticket is issued