In addition to providing formation for the unbaptized, the RCIA also makes provision for those that are already baptized and living as practicing Christians to come into full communion with the Roman Catholic Church.
The Catholic Church recognizes that those individuals that are already baptized are one with us by virtue of baptism. In reality there is very little that keeps us apart. The RCIA is designed to provide formation for baptized Christians so that they can come into full communion with the Roman Catholic Church as soon as they have received appropriate instruction. The instruction that is given in preparation for full communion varies from individual to individual as we always take into consideration the background and various life circumstances of the individual candidate for full communion. For some individuals preparing to come into full communion may closely resemble the formation that the unbaptized undergo.
Preparation for Reception into full communion with the Church begins with the Rite of Welcoming (Often in the fall, but this can take place at any time) This period of formation is an unspecified amount of time where those already baptized take time to explore what it means to be in communion with the Roman Catholic Church. This process can takes weeks or months and isn’t necessarily as long as the catechumenate, nor does it always coincide with the catechumenate.
Since the individuals taking part in this process are already Christians, it is right that their formation is suited to their needs as practicing Christians. When these Christians are ready they are received into full communion (usually with Confirmation and Eucharist) at a Sunday Mass.