A duplicated SSR code is a situation where two (or more) aircraft are squawking the same SSR code while being observable by the same controller.
The primary risk with duplicated SSR codes is that an aircraft could be mis-identified. That is, the controller would believe that a specific track corresponds to a particular aircraft while in reality this would not be the case. Consequently, the controller would issue clearances believing they will be complied with by one aircraft but in fact another one (which may also be in the airspace of another sector) will perform the action.
An example duplicated SSR code situation. The aircraft that is approaching point ENTRY is about to be transferred by ATS Unit A to ATS Unit B. If (for any reason) the other aircraft (in the middle of Unit B's airspace) calls Unit A, the controller there may misidentify it as the one approaching the boundary. Then, seeing the conflict with IVB95, they may issue a descent instruction to the new aircraft in order to solve the conflict. However, the instruction will actually be complied with by the aircraft that is exiting Unit B to the south, causing a conflict with the one flying against it 1000 ft below
SSR code distribution or automated assignment. Most ATC systems have build in functionalities for SSR code distribution and they also may assign a different code in case of a duplicate. This does not work very well in an environment with small ANSPs as there will be a lot of SSR code change if the process is not coordinated with the neighbouring units. Therefore, sometimes local agreements are made for a centralised distribution of SSR codes over several ANSPs or ATS units. For example, in Europe this is done by the Centralised Code Assignment & Management System (CCAMS).
An alternative to the above is a regional agreement for SSR code distribution, called Originating Region Code Assignment Method (ORCAM). The main idea is that different countries are allocated SSR code blocks (i.e. all codes starting with specific digits, e.g. code block 22 means all codes between 2201 and 2277). Thus, flights from different countries will be assigned different codes and within the country, the code management will be done locally. In Europe this method is used as a backup solution in case of CCAMS failure. The disadvantage of this method is that it allocates codes permanently and in case of shortage in one country (due to e.g. a peak of traffic load at a specific time period) and "surplus" in another it is not possible to use the available codes from other countries.
ICAO Doc 4444 stipulates that distinct symbols should be used for presentation of unintentionally duplicated SSR codes and/or aircraft identification that are unintentionally duplicated. This would warn air traffic controllers about the situation. They can then use an alternative identification procedure to make sure they are communicating with the correct aircraft. For example, the crew of the aircraft with a duplicated SSR code may report their position or they may use the IDENT feature of the transponder. The limitations of each procedure need to be taken into account when choosing one.
Example of a duplicate SSR warning feature. The tracks that have duplicate SSR codes have different symbols (triangles) and are bright in colour (orange) to attract controller's attention
Transitioning to Mode S airspace resolves the issue completely. This means that aircraft are identified by their downlinked callsigns which are unique. An additional benefit is that the need to change SSR codes due to duplication is also eliminated thus freeing valuable time on the frequency. The downside is that it requires, among other things, that all aircraft are equipped with Mode S transponders.