Tonight in class we're having a quiz over prepositions. One thing we want to do is avoid over-translating Greek prepositions when a compound verb is used.
Mark 1:21 is a case in point.
Here the repetition of eis doesn't necessarily indicate emphasis.
It's probably just the redundant use of the preposition that developed during the Koine period. This means that translations such as "And they entered into Capernaum" are probably over-translations. "And they entered Capernaum" suffices.
Our new subject tonight is the Greek personal pronoun. Should be tons of fun!