A duo check valve is considered a sub-category of the wafer check valve. The main characteristic of a duo-check is that the disc is split in two, and opens similar to a butterfly valve. The original/standard wafer check only has 1 disc.
A recent thread in this forum discuss what a high performance BFL valve is thread408-266485
This might be relevant.
Without retainer indicates perhaps a direct metal to metal sealing with direct contact, again the definitions for HP butterfly valves might be significant.
The above said, i must admit that I have never seen this descriptin in connection with a check valve.
On the other hand it is quite common that the description for a check valves leans towards somthing used in 'an almost similar' , but older/previous solution, without adeqatly giving the necessary fluid and operation parameters necessary to dimension and select the optimum type: eg.: correct dimension, type, placement etc. necessary to avoid pressure peaks and slamming or gulping operation, partially open valve, minimum pressure loss and soft opening and closing.
The correct solution in you case might be a metal to metal seated disc check valve, for instant of type gestra disco:
Retainerless wafer duo check valves have been designed by Goodwin UK.
It is a design that allows all internals to be assembled from the downstream side without any potential leak pass to the athmosphere.
Have a look at
