How do I help an identical twin who doesn’t like to be called a twin?

Q. Our identical twin boys (Alex and Marcus) are nearly six years old and in separate classes in junior infants since last September. However, since February, Marcus does not like being called a twin. We call them by their names at home or refer to them as the boys. The teachers are very good and refer to them only as brothers. But when other children in the school or adults comment about them being twins or how alike they are, Marcus really doesn’t like it .