Invocation is the communication paradigm of objects. It is asymmetric by definition: the invoker initiates the invocation whereas the invokee reacts to it. A proxy is a local representative of the invokee on the invoker's machine. Like the invocation, the proxy notion is asymmetric. However, when replicated objects are considered, there is a need for symmetric proxies: a regular proxy plus an invoker proxy on the invokee's machine. This paper shows that symmetric proxies are needed to implement pre-filtering of duplicated invocations. It defines both the duplicated invocation problem and the pre-filtering approach as an efficient solution. The paper presents an object model based on symmetric proxies. Finally, the implementation of the model in the GARF system is presented as well as similar works in the literature.